I would suggest you start by checking in yahoo web mail hat the email is not shown there. Account forwarding on the server side is by far the most common reason for mail to show up twice as you describe.

I checked the yahoo web email, and the duplicates do not show up there. The problem is somewhere in Thunderbird. As I mentioned previously, both of the affected addresses have been deleted and reentered, using the secure password key needed for Yahoo accounts. Is there a way to "clear cache" in TB? I suspect that there is a hidden entry that is preventing the corrected addresses to function properly.

Yes, there is a clear cache in settings, but I would not expect it to do anything in this case. But you are more than welcome to try.

Please post the troubleshoot information shown in the help menu > more troubleshooting information. (copy to clipboard and paste into a reply on the forum.)

You're right...clearing the cache did not solve the issue, but I created a workaround with a message filter. The filter on one account was set to redirect the message to the correct address. Now all I have to do is delete the duplicates in the correct address. But when I created the filter the first time, using "if a message sender contains ___, then redirect" it didn't always work. I changed the criteria to "if a message sender is _____" then it worked.

I think there is still a glitch in TB.