Zoplicate has this feature but its merge option is broken, there's still an open issue on github for it: "When merging duplicates, they are not kept in all collections but deleted" https://github.com/ChenglongMa/zoplicate/issues/158
@AymanNad, reading the github issue it seems to be (or have been ?) associated only with a narrow set of circumstances. There has been a major plugin update (v5) since the last report, but it is not clear if it addresses those circumstances (the issue is still 'open'). If you are still experiencing the problem it would be best to report it under that issue.
@tim820 I did report under the issue, the latest instance of this error was a few days back for me, so it's an active issue, even with the 'major plugin update'.
For now I have disabled the plugin in zotero, hopefully zotero devs release the native feature soon.
Upgrade Storage
https://forums.zotero.org/discussion/103977/add-ability-to-mark-multiple-items-as-not-duplicate-rather-than-merge-them
https://forums.zotero.org/discussion/123342/erroneous-duplicate-items
Quoting aborel in one of the answers:
https://github.com/ChenglongMa/zoplicate?tab=readme-ov-file#non-duplicates
https://github.com/ChenglongMa/zoplicate/issues/158
For now I have disabled the plugin in zotero, hopefully zotero devs release the native feature soon.