fix: ensure customLangExt is used only without duplicate keys

- Updated hasCustomLangExt to check for duplicate keys
- Ensures customLangExt is applied only when no duplicates exist
- Improves reliability of language extension handling

SubRenamer/Services/RenameService.cs

@@ -24,7 +24,7 @@ public void UpdateRenameTaskList(IReadOnlyList<MatchItem> matchList, Collection<
         var hasDuplicateKey = matchList.GroupBy(x => x.Key).Any(g => g.Count() > 1);
         var keepLangExt = Config.Get().KeepLangExt || hasDuplicateKey;
         var customLangExt = Config.Get().CustomLangExt.Trim();
-        var hasCustomLangExt = !string.IsNullOrEmpty(customLangExt);
+        var hasCustomLangExt = !hasDuplicateKey && !string.IsNullOrEmpty(customLangExt);
 
         foreach (var item in matchList)
         {