Gmail does not use folders, it has 'labels' instead, which are used for sorting messages within Gmail. Each label is reflected in Thunderbird as a folder. Gmail has an 'all mail' label as well as the usual 'inbox', 'sent' etc. Consequently, within Gmail every message is labelled twice: all incoming messages are labelled 'inbox' and 'all mail', all outgoing ones are labelled 'sent' and 'all mail' and so on. It's likely that you have somehow got the 'all mail' label/folder in your Thunderbird set up. There's an article here that may help you sort it out: https://support.mozilla.org/en-US/kb/thunderbird-and-gmail The section 'Gmail's particularities' is relevant.

Thanks; Most of that is over my head, along with all the search results I've read. Simply put... In my inbox, I have just my personal email address and my Gmail address, which has no drop-downs and no mail in it. Below that, my archives are the same, just personal and Gmail, and here Gmail has all my emails in it that I didn't delete. Below that, my personal email has all the folders I've made. Below that is my Gmail address, with a Gmail folder that drops down to 'important' and 'starred'. I've never used starred, and important has all my emails in it similar to archives. I don't have 'All Mail' anywhere in Thunderbird. 'All Mail' in Google has all my Gmail messages that Thunderbird has. And the only way I found it is by accidentally hovering over 'Junk'. None of the images in 'Gmail's particularities' look anything like what I have in Thunderbird or Gmail. It must be years old. So nothing I've read tells me how to fix it, but I appreciate your help.

Perhaps it should be asked where do you see the two copies? Both in the same folder, or do the two copies appear in two different folders? And if the latter, which two folders?

The two duplicates come into my Thunderbird personal inbox, and a third goes into my Thunderbird Gmail inbox.

I'd like just one to go to my personal inbox.

I just noticed that if I delete one of the two in my personal inbox, that automatically deletes the one in my Gmail inbox. And still leaves one in my personal inbox.