diff --git a/ecs.php b/ecs.php
index 964eddb..88ffd43 100644
--- a/ecs.php
+++ b/ecs.php
@@ -1,13 +1,18 @@
 <?php declare(strict_types=1);
 
 use Symplify\EasyCodingStandard\Config\ECSConfig;
+use Symplify\EasyCodingStandard\ValueObject\Set\SetList;
 
-return ECSConfig::configure()
-    ->withPaths([
+return static function (\Symplify\EasyCodingStandard\Config\ECSConfig $ecsConfig): void {
+    $ecsConfig->paths([
         __DIR__ . '/src',
         __DIR__ . '/tests',
-    ])
-    ->withSkip([
+    ]);
+    $ecsConfig->skip([
         __DIR__ . '/tests/fixtures',
-    ])
-    ->withPhpCsFixerSets(perCS20: true);
+    ]);
+    $ecsConfig->sets([SetList::PSR_12, SetList::NAMESPACES, SetList::DOCBLOCK]);
+
+    $ecsConfig->rules([
+    ]);
+};
diff --git a/src/Rules/DuplicatedArrayKeys.php b/src/Rules/DuplicatedArrayKeys.php
new file mode 100644
index 0000000..278b2f9
--- /dev/null
+++ b/src/Rules/DuplicatedArrayKeys.php
@@ -0,0 +1,80 @@
+<?php
+
+declare(strict_types=1);
+
+namespace PMarki\PHPStanRules\Rules;
+
+use PhpParser\Node;
+use PhpParser\Node\ArrayItem;
+use PhpParser\Node\Expr\Array_;
+use PHPStan\Analyser\Scope;
+use PHPStan\Rules\Rule;
+use PHPStan\Rules\RuleErrorBuilder;
+
+/**
+ * Check if keys in array are unique
+ *
+ * @implements Rule<Array_>
+ */
+class DuplicatedArrayKeys implements Rule
+{
+    public function getNodeType(): string
+    {
+        return Array_::class;
+    }
+
+    /**
+     * @return array<int, \PHPStan\Rules\RuleError>
+     */
+    public function processNode(Node $node, Scope $scope): array
+    {
+        if (!$node instanceof Array_) {
+            return [];
+        }
+
+        $errors = [];
+        $existingKeys = [];
+
+        foreach ($node->items as $item) {
+            if (!$item instanceof ArrayItem || $item->key === null) {
+                continue;
+            }
+
+            $key = $this->getKey($item->key);
+            if ($key === null) {
+                continue;
+            }
+
+            if (\in_array($key, $existingKeys, true)) {
+                $errors[] = RuleErrorBuilder::message("Array key '$key' is duplicated.")
+                    ->identifier('extraDuplicatedArrayKeys')
+                    ->line($item->key->getLine())
+                    ->build();
+                continue;
+            }
+
+            $existingKeys[] = $key;
+        }
+
+        return $errors;
+    }
+
+    private function getKey(Node $node): string|int|null
+    {
+        if ($node instanceof Node\Scalar\String_) {
+            $arr = [$node->value => 1];
+
+            return \array_key_first($arr);
+        }
+
+        if ($node instanceof Node\Scalar\Int_) {
+            return $node->value;
+        }
+
+        if ($node instanceof Node\Scalar\Float_) {
+            return (int) $node->value;
+        }
+
+        return null;
+    }
+}
diff --git a/src/Rules/ProtectedPropertyRule.php b/src/Rules/ProtectedPropertyRule.php
index fe6ae8b..bf6c637 100644
--- a/src/Rules/ProtectedPropertyRule.php
+++ b/src/Rules/ProtectedPropertyRule.php
@@ -6,8 +6,8 @@
 
 use PhpParser\Node;
 use PhpParser\Node\Stmt\Property;
-use PHPStan\Node\ClassPropertyNode;
 use PHPStan\Analyser\Scope;
+use PHPStan\Node\ClassPropertyNode;
 use PHPStan\Rules\Rule;
 use PHPStan\Rules\RuleErrorBuilder;
 
diff --git a/tests/DuplicatedArrayKeysTest.php b/tests/DuplicatedArrayKeysTest.php
new file mode 100644
index 0000000..904b647
--- /dev/null
+++ b/tests/DuplicatedArrayKeysTest.php
@@ -0,0 +1,38 @@
+<?php
+
+declare(strict_types=1);
+
+namespace PMarki\PHPStanRules\Tests;
+
+use PHPStan\Rules\Rule;
+use PHPStan\Testing\RuleTestCase;
+use PMarki\PHPStanRules\Rules\DuplicatedArrayKeys;
+
+class DuplicatedArrayKeysTest extends RuleTestCase
+{
+    public function testNoAbstractClass(): void
+    {
+        $this->analyse(
+            [__DIR__ . '/fixtures/DuplicatedArrayKeys.php'],
+            [
+                [
+                    "Array key 'key1' is duplicated.",
+                    6,
+                ],
+                [
+                    "Array key '2' is duplicated.",
+                    12,
+                ],
+                [
+                    "Array key 'nested-key2' is duplicated.",
+                    30,
+                ],
+            ],
+        );
+    }
+
+    protected function getRule(): Rule
+    {
+        return new DuplicatedArrayKeys();
+    }
+}
diff --git a/tests/fixtures/DuplicatedArrayKeys.php b/tests/fixtures/DuplicatedArrayKeys.php
new file mode 100644
index 0000000..2b476e0
--- /dev/null
+++ b/tests/fixtures/DuplicatedArrayKeys.php
@@ -0,0 +1,38 @@
+<?php
+
+$array = [
+    'key1' => 'value1',
+    'key2' => 'value2',
+    'key1' => 'duplicate',
+];
+
+$array = [
+    1 => 'value1',
+    2 => 'value2',
+    2 => 'value3',
+];
+
+$array = [
+    'key1' => 'value1',
+    'key2' => 'value2',
+    'key3' => [
+        'key1' => 'value1',
+        'key2' => 'value2',
+    ]
+];
+
+$array = [
+    'key1' => 'value1',
+    'key2' => 'value2',
+    'key3' => [
+        'nested-key1' => 'value1',
+        'nested-key2' => 'value2',
+        'nested-key2' => 'duplicated',
+    ]
+];
+
+$array = [
+    'value1',
+    'value2',
+    'value1',
+];