Why is Apple Mail on MacBook saving multiple copies?

Restarted MacBook after changes to Preferences in Mail. Multiple copies still being created in Archive. The Sent mail remains after I delete the multiple copies in Archive.

I now have over 6000 emails in Archive which I never intentionally place there. I have the habit of leaving Sent mail in the Sent mail folder or moving it to one of the On My Mac folders I created.

I'm ready to buy a new MacBook but reluctant to do so while I have this issue and Apple stores are closed. I usually go to the one in Tysons Corner, Virginia.

Archive still storing multiple copies of Sent Mail.

Drafts Mailbox set to Drafts on iCloud. Changed to Drafts On My Mac.

I believe the problem began on April 8, 2020 after I did a Security Update.

... but why multiple 3 to 10 copies?

Could it be that Mail keeps saving copies to allow "Handoff" to another registered device, and then deletes the copies after a certain period?

More weird stuff in Mail. Created a Draft, verified that it was Saved in the Draft folder. A minute later the Draft folder was empty and the Draft was found in Archive.

It seems that multiple copies of Sent mail are no longer being saved to Archive. Drafts still disappearing from Drafts folder into Archive.