I'm not seeing anything similar on Thunderbird at my location. The one thought I had was maybe it is an issue with the mail server Can you connect to a second different server and see whether the problem shows up there as well? Then, you should be able to localize the problem to server or app.

If Thunderbird is downloading multiple copies of incoming messages, it's likely due to incorrect account settings, particularly related to the "Leave messages on server" option, especially if you're using a POP3 account; check your settings to ensure messages are deleted from the server after being downloaded once, and consider compacting your mail folders to clean up any potential indexing issues.

Key points to check:

Account settings:

Go to "Tools > Account Settings" in Thunderbird.

Select your email account and navigate to "Server Settings".

Ensure "Leave messages on server" is unchecked if you want to prevent duplicates on a POP3 account.

Check for corrupted data:

Compact folders: Regularly compact your mail folders to clean up potential indexing issues.

Delete "popstate.dat" file (for POP accounts): If the issue persists, try deleting the "popstate.dat" file located within your Thunderbird profile folder.

Other potential causes:

Multiple email clients accessing the same account: Check if another email program is accessing the same account simultaneously.

Server issues: Sometimes, the issue might be related to your email provider's server experiencing temporary glitches.

Add-ons interference: If you have recently installed any email management add-ons, try disabling them to see if they are causing the problem.

What to do if the problem persists:

Rebuild the mail index:
Try rebuilding the mail index by going to "Help > Troubleshooting Information > Show Folder" and then deleting the "global" file.

Create a new profile:
As a last resort, consider creating a new Thunderbird profile and migrating your data over.

Check for updates:
Make sure you have the latest version of Thunderbird installed, as updates often include bug fixes.

The problem seems to be fixed.

That box was checked, that is why the problems.

Thanks so much.

I have sent and received multiple copies when the email server is lost in space.

Mail servers queue up outgoing and incoming emails, and when they have problems reaching destinations, they retry.

And you never know what was the hangup: Storage slow or down, like AWS (Amazon) or some hangup at Cloudflare or a nasty network outage at who knows where.

I keep two email accounts, and use various apps to handle mail, so at the very least, I can get a better handle on problems.

If Thunderbird works at all (as say it does) then it's probably configured correctly.

From LoveLoogie and Usonian seem to be the best explanations for this behaviour. I'm a long term Thunderbird user and always use POP3 simply because I don't need to access mail from anywhere but my desktop computer.

Who is your email provider? Are there any traces online about problems they are having?

Is your account set to delete messages after downloading?