Is one an alias account, or are they each fully independent accounts? If each is a separate account, one possibility is that one is forwarded to the other at the server level. Another possibility is that you have have an overlooked filter. Solving this will require a detailed review of the accounts online and the server settings. You say 'recently.' Did they work properly previously? What changes were made?

They are 2 fully independent account with the same provider. My service provider is Rogers (I'm located in Toronto, Canada), and my first email is myname@rogers.com, my second email is my.name@rogers.com, with the only difference being the "." separating my initials from my last name. Rogers also uses "Yahoo" mail for their email service, as when I check my webmail the address is "mail.yahoo.com". This all started when I switched my Rogers Cable in my house to "Rogers Ignite", and immediately after I started getting the messages in both account inboxes, but only within Thunderbird. How would I know if one account is forwarding to the other at the server level? Thanks

Does this happen for the other account, or just the primary? That is, does the my.name account only receive my.name mail, but the myname account receives both myname and my.name? One possible test would be to check mail for myname from online yahoo site. Do the two accounts have different passwords? If anything is happening, it will be at server level. Despite its many strengths, Thunderbird is stupid (which is what you want); it sends server setup info when requesting mail, nothing more. And it retrieves whatever the server sends back. You could also check by temporarily removing password from my.name and then check mail on myname.

I don't believe it does it going the other way, but when I tried sending an email to my.name account Thunderbird did not download from the server, but it did make it on the yahoo webmail. Do you think I should try deleting that account on Thunderbird and starting fresh?