Most often, a mail forwarding loop prevents you from receiving any mail, causing mail sent to you to bounce back to the sender. However, if you receive many copies of every email message, a forwarding loop may be the cause of this as well.

Greetings, James

Thank you for your reply. How do I deal with a "forwarding loop" to avoid the problem ?

You need to check with whomever is your email provider and ask for help on why it's doing duplicate emails. Firefox is just a Browser to get to the email service provider.

I contacted my service provider (Bell), they claim the problem is with Thunderbird email. I was guided to their Bell email where there was no evidence of any duplication of messages. I would appreciate assistance to eliminate the duplication problem.

I have this problem too and it only started about a month ago when I upgraded Thunderbird to a new release.

My email comes from my own email accounts hosted at X10Premium and there is only 1 copy of each email on the server (I leave them there for 7-14 days after they arrive as a backup).

It seems Thunderbird is now reading and downloading the emails, marking them as read on the server, but then regularly re-downloading them, so I have multiple copies of every email until it's deleted from the server.

It's also sometimes downloading 2 copies at a time.

Nothing has changed on my server as far as I am aware and this is happening on every account.

It's very frustrating and I hope the problem can be identified and fixed soon. Happy to provide any information that might help.