It's often a sign that the format or structure of the email message is faulty and doesn't allow the message to be "signed off" as successfully downloaded. So on the next connection, Thunderbird re-attempts what it assumes to be an incomplete or interrupted download. By going to webmail and removing the offending message, you allow Thunderbird to get on with what's left on the server.

Is there any pattern? Is it a particular sender, or senders using a particular email client or service?

Also, is it both yahoo and gmail accounts? And is the failing account defined as imap? or pops?

Than ks for replies,

Yahoo is the main problem as I don't use the gmail account that often. I use Pop server settings for Yahoo and Imap for gmail. There are several sources that tend to present the problem while I do receive some mail from other sources with attachments that don't cause the problem. Zenos, your comment makes sense. However I also download my emails onto my android phone and tablet without any problem ever. Is it possible for me to use imap settings for yahoo?

This sounds like and anti virus issue to me. try disabling incoming mail scanning. Often anti virus program choke and die on larger attachments/attachments of certain types (PDF get mentioned often) and that causes the connection to die as well. Thunderbird has no idea if the download was complete or not so next get mail sees the mail downloaded again.

I tried turning off email scanner in anti virus but it made no difference. I have a new email with attachment still continually downloading from a safe sender who I have not received from before which makes me doubt that the format from the senders is faulty and maybe there is a fault occurring in the downloading process that prevents Thunderbird signing off on the download. Thanks for those who have offered their support so far.