fix: fixing possible duplicate insertion

Getting errors in the log for duplicate insertions into the TwitchUser
table. This is the only place here I can see duplicates.

Author: ProgrammingByPermutation

src/Nullinside.Api.TwitchBot/Model/NullinsideContextExtensions.cs

@@ -161,7 +161,7 @@ private static async Task<int> UpdateOAuthInDatabase(this INullinsideContext db,
   /// <param name="stoppingToken">The stopping token.</param>
   public static async Task SaveTwitchBans(this INullinsideContext db, string channelId,
     IEnumerable<(string Id, string Username)> bannedUsers, string reason, CancellationToken stoppingToken = new()) {
-    List<string> banUserIds = bannedUsers.Select(b => b.Id).ToList();
+    List<string> banUserIds = bannedUsers.Select(b => b.Id).ToHashSet().ToList();
     HashSet<string?> existingUsers = db.TwitchUser
       .AsNoTracking()
       .Where(u => null != u.TwitchId && banUserIds.Contains(u.TwitchId))