We have been seeing this issue with Outlook.com POP mail accounts since the release of Windows 10.

Microsoft's preferred solution.... Use IMAP not POP.

Another options is to set Thunderbird to get mail every 30 or 40 minutes not every 15.

Another is to turn off the option on their web site that moves the mail to another folder when Thunderbird deletes it. (it is a lengthy delay here at the root of the problem.

Thanks Matt.

That must be why I am having the problem. My email was supposed to go to the POP folder and it is not doing it.

But now all 50,000 messages are being downloaded all over again. What a pain!

Here are some other threads about receiving duplicate email for the original user: https://support.mozilla.org/en-US/questions/1061656 https://support.mozilla.org/en-US/questions/1005565 https://support.mozilla.org/en-US/questions/1139782

The first mentions an add-on to delete the duplicates. I am about to try it: https://addons.mozilla.org/en-us/thunderbird/addon/remove-duplicate-messages/