An email keeps repeating (288 times)

Please explain what you're doing and what happens. What is your account type - POP or IMAP?

I have tried to uninstall Thunderbird

Why would you want to do that in the first place?

Christ1, My account is POP. I had thought if I uninstalled Thunderbird and then reinstalled it, The message might have disappeared. I read that deleting the "popstate.dat" file might get rid of the email, but that didn't seem to work.

Go to the source of the problem, your inbox on your providers server and delete the message or move it out of the inbox and into another folder.

This doesn't work. There are still over 300 copies of the message trying to down load.

Unity 12

There are 300 copies of this message in your inbox when you use web mail and look at your account? If so then you have a problem that Thunderbird cannot fix. Thunderbird downloads messages in your servers inbox. It does not create them.

Did you really go to your providers web mail page and look at your account?

I guess you were right. I haven't found the web mail page. I thought to find it through "tools" but could find nothing relating to my account.

Any other suggestions?

Your providers web mail page has nothing to do with Thunderbird. You navigate to that page using an Internet browser and log into your account there. I have no idea who your provider is so I cannot help you there. Mine is Comcast so I go to comcast.net and click the email tab.