Question [Q] Duplicate emails

I have for years had multiple email accounts all linked to the standard Samsung email app, and all was well. I have now realised that the `sent` folder contains thousands of copies of the emails, but the recipients say they have only received one. I have tried clearing all the app data etc, deleting the extra copies etc, but as soon as I look in the sent folder they start reappearing again.. I have tried K9 mail as well and the extra copies show up there too.
Any ideas why this is happening and how do I rectify the problem?