You have to setup a filter in Thunderbird to forward mail. If you do not have a filter you must have done it elsewhere. Check your providers webmail site and see if you did it there.

I looked at both Thunderbird and Yahoo and their was not a filter set on either. I do remember when I did "something" to set it up, but I can't seem to find how I did it, lol

This page explains how to link or unlink other accounts, if that's what you did.

maybe I am not explaining the problem correctly.

I have email address abc@123.com every email that comes to this address is ALSO sent to another email address def@456.com

Each email rc'd is mirrored/copied and sent to the second email address(at the same time). I originally set this up after I had accidentally deleted a bunch of emails that I needed and was unable to retrieve them. So I set it up to automatically copy and send every email that comes to first email address to the second address.

I no longer need this to happen and can't figure out how to undo it! It is now getting annoying to get so many emails in duplicate, lol

thanks for the help

See https://uk.help.yahoo.com/kb/new-mail-for-desktop/SLN29133.html for Yahoo's auto-forwarding settings. You should be able to find and remove the forwarding address from your Yahoo's mailbox thus stopping the automatic forwarding.