diff --git a/docs/02-admin/04-running-mbin/05-cli.md b/docs/02-admin/04-running-mbin/05-cli.md
index 52243c5a0..2faa9b0f0 100644
--- a/docs/02-admin/04-running-mbin/05-cli.md
+++ b/docs/02-admin/04-running-mbin/05-cli.md
@@ -162,6 +162,23 @@ php bin/console mbin:user:unsub <username>
 Arguments:
 - `username`: the user from which to remove all local followers.
 
+### Fix user duplicates
+
+This command allows you to fix duplicate usernames. There is a unique index on the usernames, but it is case-sensitive.
+This command will go through all the users with duplicate case-insensitive usernames,
+where the username is not part of the public id (meaning the original URL) and update them from the remote server.
+After that it will go through the rest of the duplicates and ask you whether you want to merge matching pairs.
+
+Usage:
+
+```bash
+php bin/console mbin:users:fix-duplicates [--dry-run]
+```
+
+Options:
+- `--dry-run`: don't change anything in the DB
+
+
 ## Magazine Management
 
 ### Magazine-Create
diff --git a/src/Command/ActorUpdateCommand.php b/src/Command/ActorUpdateCommand.php
index 5cb7df5b4..d3fa88154 100644
--- a/src/Command/ActorUpdateCommand.php
+++ b/src/Command/ActorUpdateCommand.php
@@ -15,6 +15,7 @@
 use Symfony\Component\Console\Output\OutputInterface;
 use Symfony\Component\Console\Style\SymfonyStyle;
 use Symfony\Component\Messenger\MessageBusInterface;
+use Symfony\Component\Messenger\Stamp\TransportNamesStamp;
 
 #[AsCommand(
     name: 'mbin:actor:update',
@@ -45,15 +46,15 @@ protected function execute(InputInterface $input, OutputInterface $output): int
         $force = (bool) $input->getOption('force');
 
         if ($userArg) {
-            $this->bus->dispatch(new UpdateActorMessage($userArg, $force));
+            $this->bus->dispatch(new UpdateActorMessage($userArg, $force), [new TransportNamesStamp('sync')]);
         } elseif ($input->getOption('users')) {
             foreach ($this->repository->findRemoteForUpdate() as $u) {
-                $this->bus->dispatch(new UpdateActorMessage($u->apProfileId, $force));
+                $this->bus->dispatch(new UpdateActorMessage($u->apProfileId, $force), [new TransportNamesStamp('sync')]);
                 $io->info($u->username);
             }
         } elseif ($input->getOption('magazines')) {
             foreach ($this->magazineRepository->findRemoteForUpdate() as $u) {
-                $this->bus->dispatch(new UpdateActorMessage($u->apProfileId, $force));
+                $this->bus->dispatch(new UpdateActorMessage($u->apProfileId, $force), [new TransportNamesStamp('sync')]);
                 $io->info($u->name);
             }
         }
diff --git a/src/Command/FixUserDuplicatesCommand.php b/src/Command/FixUserDuplicatesCommand.php
new file mode 100644
index 000000000..f200c3eaa
--- /dev/null
+++ b/src/Command/FixUserDuplicatesCommand.php
@@ -0,0 +1,377 @@
+<?php
+
+declare(strict_types=1);
+
+namespace App\Command;
+
+use App\Entity\User;
+use App\Exception\InvalidApPostException;
+use App\Repository\InstanceRepository;
+use App\Repository\UserRepository;
+use App\Service\ActivityPub\ApHttpClientInterface;
+use App\Service\ActivityPubManager;
+use App\Service\MentionManager;
+use Doctrine\DBAL\ParameterType;
+use Doctrine\ORM\EntityManagerInterface;
+use Psr\Cache\InvalidArgumentException;
+use Symfony\Component\Console\Attribute\AsCommand;
+use Symfony\Component\Console\Command\Command;
+use Symfony\Component\Console\Input\InputInterface;
+use Symfony\Component\Console\Input\InputOption;
+use Symfony\Component\Console\Output\OutputInterface;
+use Symfony\Component\Console\Style\SymfonyStyle;
+
+#[AsCommand(
+    name: 'mbin:users:fix-duplicates',
+    description: 'Guided tour to fix duplicate usernames'
+)]
+class FixUserDuplicatesCommand extends Command
+{
+    public function __construct(
+        private readonly EntityManagerInterface $entityManager,
+        private readonly ApHttpClientInterface $apHttpClient,
+        private readonly UserRepository $userRepository,
+        private readonly ActivityPubManager $activityPubManager,
+        private readonly InstanceRepository $instanceRepository,
+        private readonly MentionManager $mentionManager,
+    ) {
+        parent::__construct();
+    }
+
+    protected function configure(): void
+    {
+        $this->addOption('--dry-run', null, InputOption::VALUE_NONE, 'Do not make any changes to the database');
+    }
+
+    protected function execute(InputInterface $input, OutputInterface $output): int
+    {
+        $io = new SymfonyStyle($input, $output);
+
+        $sql = 'SELECT
+    LOWER(username) AS normalized_username,
+    COUNT(*)        AS duplicate_count,
+    json_agg(id)   AS user_ids,
+    json_agg(username) AS usernames,
+    json_agg(ap_profile_id) as urls
+FROM "user"
+WHERE application_status = \'Approved\' AND ap_id IS NOT NULL
+GROUP BY LOWER(username)
+HAVING COUNT(*) > 1
+ORDER BY duplicate_count DESC';
+
+        $io->writeln('Gathering duplicate users');
+        $result = $this->entityManager->getConnection()->prepare($sql)->executeQuery()->fetchAllAssociative();
+
+        $usersToUpdate = [];
+        $usersToMerge = [];
+        $io->writeln('Determining which users to update');
+        $dryRun = \boolval($input->getOption('dry-run'));
+        foreach ($result as $row) {
+            $username = $this->mentionManager->getUsername($row['normalized_username']);
+            $urls = json_decode($row['urls']);
+            // if the URL contains the string (after removing the host), the username is probably right
+            $urlsToUpdate = array_filter($urls, fn (string $url) => !str_contains(str_replace('https://'.parse_url($url, PHP_URL_HOST), '', strtolower($url)), $username));
+
+            foreach ($urlsToUpdate as $url) {
+                $usersToUpdate = array_merge($usersToUpdate, $this->getUsersToUpdateFromUrl($url, $io, $dryRun));
+            }
+
+            $referenceUrl = strtolower($urls[0]);
+            $pairShouldBeMerged = true;
+            foreach ($urls as $url) {
+                if ($referenceUrl !== strtolower($url)) {
+                    $pairShouldBeMerged = false;
+                    break;
+                }
+            }
+            if ($pairShouldBeMerged) {
+                $usersToMerge[] = [json_decode($row['user_ids'])];
+            }
+        }
+        $io->writeln('Updating users from URLs: '.implode(', ', $usersToUpdate));
+        foreach ($usersToUpdate as $url) {
+            if (!$dryRun) {
+                $io->writeln("Updating actor $url");
+                $this->activityPubManager->updateActor($url);
+            } else {
+                $io->writeln("Would have updated actor $url");
+            }
+        }
+
+        foreach ($usersToMerge as $userPairs) {
+            $users = $this->userRepository->findBy(['id' => $userPairs]);
+            $userString = implode(', ', array_map(fn (User $user) => "'$user->username' ($user->apProfileId)", $users));
+            $answer = $io->ask("Should these users get merged: $userString", 'yes');
+            if ('yes' === $answer) {
+                $this->mergeRemoteUsers($users, $io, $dryRun);
+            }
+        }
+
+        return Command::SUCCESS;
+    }
+
+    private function getUsersToUpdateFromUrl(string $url, SymfonyStyle $io, bool $dryRun): array
+    {
+        $user = $this->userRepository->findOneBy(['apProfileId' => $url]);
+        if (!$user) {
+            return [];
+        }
+
+        if ($user->isDeleted || $user->isSoftDeleted() || $user->isTrashed() || $user->markedForDeletionAt) {
+            // deleted users do not get updated, thus they can cause non-unique violations if they also have a wrong username
+            if (!$dryRun) {
+                $answer = $io->ask("Do you want to purge user '$user->username' ($user->apProfileId)? They are already deleted.", 'yes');
+                if ('yes' === strtolower($answer)) {
+                    $this->purgeUser($user);
+                }
+            } else {
+                $io->writeln("Would have asked whether '$user->username' ($user->apProfileId) should be purged");
+            }
+
+            return [];
+        }
+
+        $instance = $this->instanceRepository->findOneBy(['domain' => $user->apDomain]);
+        if ($instance && ($instance->isBanned || $instance->isDead())) {
+            if (!$dryRun) {
+                $answer = $io->ask("The instance $instance->domain is either dead or banned, should the user '$user->username' ($user->apProfileId) be purged?", 'yes');
+                if ('yes' === strtolower($answer)) {
+                    $this->purgeUser($user);
+                }
+            } else {
+                $io->writeln("Would have asked whether '$user->username' ($user->apProfileId) should be purged");
+            }
+
+            return [];
+        }
+
+        $io->writeln("fetching remote object for '$user->username' ($user->apProfileId)");
+        $actorObject = $this->apHttpClient->getActorObject($url);
+        if (!$actorObject) {
+            if (!$dryRun) {
+                $this->purgeUser($user);
+            } else {
+                $io->writeln("Would have purged user '$user->username' ('$user->apProfileId'), because we didn't get a response from the server");
+            }
+
+            return [];
+        } elseif ('Tombstone' === $actorObject['type']) {
+            if (!$dryRun) {
+                $this->purgeUser($user);
+            } else {
+                $io->writeln("Would have purged user '$user->username' ('$user->apProfileId'), because it is deleted on the remote server");
+            }
+
+            return [];
+        }
+
+        $domain = parse_url($url, PHP_URL_HOST);
+        $newUsername = '@'.$actorObject['preferredUsername'].'@'.$domain;
+        // if there already is a user with the username that is supposed to be on the current one,
+        // we have to update that user before the current one to avoid non-unique violations
+        $existingUsers = $this->userRepository->findBy(['username' => $newUsername]);
+        $result = [];
+        foreach ($existingUsers as $existingUser) {
+            if ($existingUser) {
+                if ($user->getId() === $existingUser->getId()) {
+                    continue;
+                }
+                $additionalUrls = $this->getUsersToUpdateFromUrl($existingUser->apProfileId, $io, $dryRun);
+
+                $result = array_merge($additionalUrls, $result);
+            }
+        }
+        $result[] = $url;
+
+        return $result;
+    }
+
+    /**
+     * @throws \Doctrine\DBAL\Exception
+     */
+    private function purgeUser(User $user): void
+    {
+        $stmt = $this->entityManager->getConnection()
+            ->prepare('DELETE FROM "user" WHERE id = :id');
+        $stmt->bindValue('id', $user->getId(), ParameterType::INTEGER);
+        $stmt->executeStatement();
+    }
+
+    /**
+     * This replaces all the references of one user with all the others. The main user the others are merged into
+     * is determined by the exact match of the 'id' gathered from the URL in `$user->apProfileId`.
+     *
+     * @param User[] $users
+     *
+     * @throws InvalidApPostException
+     * @throws \Doctrine\DBAL\Exception
+     * @throws InvalidArgumentException
+     */
+    private function mergeRemoteUsers(array $users, SymfonyStyle $io, bool $dryRun): void
+    {
+        if (0 === \count($users)) {
+            return;
+        }
+
+        $actorObject = $this->apHttpClient->getActorObject($users[0]->apProfileId);
+        $mainUser = array_first(array_filter($users, fn (User $user) => $user->apProfileId === $actorObject['id']));
+
+        if (!$mainUser) {
+            $io->warning(\sprintf('Could not find an exact match for %s in the users %s', $actorObject['id'], implode(', ', array_map(fn (User $user) => $user->apProfileId, $users))));
+
+            return;
+        }
+
+        foreach ($users as $user) {
+            if ($mainUser->getId() === $user->getId()) {
+                continue;
+            }
+
+            if ($dryRun) {
+                $io->writeln("Would have merged '$user->username' ('$user->apProfileId') into '$mainUser->username' ('$mainUser->apProfileId')");
+                continue;
+            }
+
+            $io->writeln("Merging '$user->username' ('$user->apProfileId') into '$mainUser->username' ('$mainUser->apProfileId')");
+            $conn = $this->entityManager->getConnection();
+
+            $stmt = $conn->prepare('UPDATE activity SET user_actor_id = :main WHERE user_actor_id = :oldId');
+            $stmt->bindValue(':main', $mainUser->getId(), ParameterType::INTEGER);
+            $stmt->bindValue(':oldId', $user->getId(), ParameterType::INTEGER);
+            $stmt->executeStatement();
+
+            $stmt = $conn->prepare('UPDATE entry SET user_id = :main WHERE user_id = :oldId');
+            $stmt->bindValue(':main', $mainUser->getId(), ParameterType::INTEGER);
+            $stmt->bindValue(':oldId', $user->getId(), ParameterType::INTEGER);
+            $stmt->executeStatement();
+
+            $stmt = $conn->prepare('UPDATE entry_comment SET user_id = :main WHERE user_id = :oldId');
+            $stmt->bindValue(':main', $mainUser->getId(), ParameterType::INTEGER);
+            $stmt->bindValue(':oldId', $user->getId(), ParameterType::INTEGER);
+            $stmt->executeStatement();
+
+            $stmt = $conn->prepare('UPDATE post SET user_id = :main WHERE user_id = :oldId');
+            $stmt->bindValue(':main', $mainUser->getId(), ParameterType::INTEGER);
+            $stmt->bindValue(':oldId', $user->getId(), ParameterType::INTEGER);
+            $stmt->executeStatement();
+
+            $stmt = $conn->prepare('UPDATE post_comment SET user_id = :main WHERE user_id = :oldId');
+            $stmt->bindValue(':main', $mainUser->getId(), ParameterType::INTEGER);
+            $stmt->bindValue(':oldId', $user->getId(), ParameterType::INTEGER);
+            $stmt->executeStatement();
+
+            $stmt = $conn->prepare('UPDATE entry_vote SET user_id = :main WHERE user_id = :oldId');
+            $stmt->bindValue(':main', $mainUser->getId(), ParameterType::INTEGER);
+            $stmt->bindValue(':oldId', $user->getId(), ParameterType::INTEGER);
+            $stmt->executeStatement();
+
+            $stmt = $conn->prepare('UPDATE entry_comment_vote SET user_id = :main WHERE user_id = :oldId');
+            $stmt->bindValue(':main', $mainUser->getId(), ParameterType::INTEGER);
+            $stmt->bindValue(':oldId', $user->getId(), ParameterType::INTEGER);
+            $stmt->executeStatement();
+
+            $stmt = $conn->prepare('UPDATE post_vote SET user_id = :main WHERE user_id = :oldId');
+            $stmt->bindValue(':main', $mainUser->getId(), ParameterType::INTEGER);
+            $stmt->bindValue(':oldId', $user->getId(), ParameterType::INTEGER);
+            $stmt->executeStatement();
+
+            $stmt = $conn->prepare('UPDATE post_comment_vote SET user_id = :main WHERE user_id = :oldId');
+            $stmt->bindValue(':main', $mainUser->getId(), ParameterType::INTEGER);
+            $stmt->bindValue(':oldId', $user->getId(), ParameterType::INTEGER);
+            $stmt->executeStatement();
+
+            $stmt = $conn->prepare('UPDATE favourite SET user_id = :main WHERE user_id = :oldId');
+            $stmt->bindValue(':main', $mainUser->getId(), ParameterType::INTEGER);
+            $stmt->bindValue(':oldId', $user->getId(), ParameterType::INTEGER);
+            $stmt->executeStatement();
+
+            $stmt = $conn->prepare('UPDATE message_thread_participants SET user_id = :main WHERE user_id = :oldId');
+            $stmt->bindValue(':main', $mainUser->getId(), ParameterType::INTEGER);
+            $stmt->bindValue(':oldId', $user->getId(), ParameterType::INTEGER);
+            $stmt->executeStatement();
+
+            $stmt = $conn->prepare('UPDATE message SET sender_id = :main WHERE sender_id = :oldId');
+            $stmt->bindValue(':main', $mainUser->getId(), ParameterType::INTEGER);
+            $stmt->bindValue(':oldId', $user->getId(), ParameterType::INTEGER);
+            $stmt->executeStatement();
+
+            $stmt = $conn->prepare('UPDATE magazine_block SET user_id = :main WHERE user_id = :oldId');
+            $stmt->bindValue(':main', $mainUser->getId(), ParameterType::INTEGER);
+            $stmt->bindValue(':oldId', $user->getId(), ParameterType::INTEGER);
+            $stmt->executeStatement();
+
+            $stmt = $conn->prepare('UPDATE magazine_log SET user_id = :main WHERE user_id = :oldId');
+            $stmt->bindValue(':main', $mainUser->getId(), ParameterType::INTEGER);
+            $stmt->bindValue(':oldId', $user->getId(), ParameterType::INTEGER);
+            $stmt->executeStatement();
+
+            $stmt = $conn->prepare('UPDATE magazine_log SET acting_user_id = :main WHERE acting_user_id = :oldId');
+            $stmt->bindValue(':main', $mainUser->getId(), ParameterType::INTEGER);
+            $stmt->bindValue(':oldId', $user->getId(), ParameterType::INTEGER);
+            $stmt->executeStatement();
+
+            $stmt = $conn->prepare('UPDATE magazine_subscription SET user_id = :main WHERE user_id = :oldId');
+            $stmt->bindValue(':main', $mainUser->getId(), ParameterType::INTEGER);
+            $stmt->bindValue(':oldId', $user->getId(), ParameterType::INTEGER);
+            $stmt->executeStatement();
+
+            $stmt = $conn->prepare('UPDATE moderator SET user_id = :main WHERE user_id = :oldId');
+            $stmt->bindValue(':main', $mainUser->getId(), ParameterType::INTEGER);
+            $stmt->bindValue(':oldId', $user->getId(), ParameterType::INTEGER);
+            $stmt->executeStatement();
+
+            $stmt = $conn->prepare('UPDATE moderator SET added_by_user_id = :main WHERE added_by_user_id = :oldId');
+            $stmt->bindValue(':main', $mainUser->getId(), ParameterType::INTEGER);
+            $stmt->bindValue(':oldId', $user->getId(), ParameterType::INTEGER);
+            $stmt->executeStatement();
+
+            $stmt = $conn->prepare('UPDATE notification_settings SET target_user_id = :main WHERE target_user_id = :oldId');
+            $stmt->bindValue(':main', $mainUser->getId(), ParameterType::INTEGER);
+            $stmt->bindValue(':oldId', $user->getId(), ParameterType::INTEGER);
+            $stmt->executeStatement();
+
+            $stmt = $conn->prepare('UPDATE report SET reported_id = :main WHERE reported_id = :oldId');
+            $stmt->bindValue(':main', $mainUser->getId(), ParameterType::INTEGER);
+            $stmt->bindValue(':oldId', $user->getId(), ParameterType::INTEGER);
+            $stmt->executeStatement();
+
+            $stmt = $conn->prepare('UPDATE report SET reporting_id = :main WHERE reporting_id = :oldId');
+            $stmt->bindValue(':main', $mainUser->getId(), ParameterType::INTEGER);
+            $stmt->bindValue(':oldId', $user->getId(), ParameterType::INTEGER);
+            $stmt->executeStatement();
+
+            $stmt = $conn->prepare('UPDATE user_block SET blocker_id = :main WHERE blocker_id = :oldId');
+            $stmt->bindValue(':main', $mainUser->getId(), ParameterType::INTEGER);
+            $stmt->bindValue(':oldId', $user->getId(), ParameterType::INTEGER);
+            $stmt->executeStatement();
+
+            $stmt = $conn->prepare('UPDATE user_block SET blocked_id = :main WHERE blocked_id = :oldId');
+            $stmt->bindValue(':main', $mainUser->getId(), ParameterType::INTEGER);
+            $stmt->bindValue(':oldId', $user->getId(), ParameterType::INTEGER);
+            $stmt->executeStatement();
+
+            $stmt = $conn->prepare('UPDATE user_follow SET follower_id = :main WHERE follower_id = :oldId');
+            $stmt->bindValue(':main', $mainUser->getId(), ParameterType::INTEGER);
+            $stmt->bindValue(':oldId', $user->getId(), ParameterType::INTEGER);
+            $stmt->executeStatement();
+
+            $stmt = $conn->prepare('UPDATE user_follow SET following_id = :main WHERE following_id = :oldId');
+            $stmt->bindValue(':main', $mainUser->getId(), ParameterType::INTEGER);
+            $stmt->bindValue(':oldId', $user->getId(), ParameterType::INTEGER);
+            $stmt->executeStatement();
+
+            $stmt = $conn->prepare('UPDATE user_follow_request SET follower_id = :main WHERE follower_id = :oldId');
+            $stmt->bindValue(':main', $mainUser->getId(), ParameterType::INTEGER);
+            $stmt->bindValue(':oldId', $user->getId(), ParameterType::INTEGER);
+            $stmt->executeStatement();
+
+            $stmt = $conn->prepare('UPDATE user_follow_request SET following_id = :main WHERE following_id = :oldId');
+            $stmt->bindValue(':main', $mainUser->getId(), ParameterType::INTEGER);
+            $stmt->bindValue(':oldId', $user->getId(), ParameterType::INTEGER);
+            $stmt->executeStatement();
+
+            $io->writeln("Purging user '$user->username' ('$user->apProfileId')");
+            $this->purgeUser($user);
+        }
+    }
+}
diff --git a/src/Service/ActivityPubManager.php b/src/Service/ActivityPubManager.php
index 568a7c02f..e13cd92aa 100644
--- a/src/Service/ActivityPubManager.php
+++ b/src/Service/ActivityPubManager.php
@@ -384,6 +384,7 @@ private function updateUser(string $actorUrl): ?User
             $user->type = $actor['type'] ?? 'Person';
             $user->apInboxUrl = $actor['endpoints']['sharedInbox'] ?? $actor['inbox'];
             $user->apDomain = parse_url($actor['id'], PHP_URL_HOST);
+            $user->username = '@'.($actor['preferredUsername'] ?? $user->username).'@'.$user->apDomain;
             $user->apFollowersUrl = $actor['followers'] ?? null;
             $user->apAttributedToUrl = $actor['attributedTo'] ?? null;
             $user->apPreferredUsername = $actor['preferredUsername'] ?? null;