Had the same issue (some messages doubled on few consecutive updates few months ago so I had up to 8 copies of the old messages).

I finally have found an app (free) that can easily remove duplicate messages - "BackupSMS" (by kamma)

https://play.google.com/store/apps/details?id=cz.kamma.backupsms

After removing duplicate messages I also did a resync in Textra (Settings -> About -> Resync Textra) and now I don't have duplicates.

Will report the Textra version if they duplicate again.