You have a Gmail account. You can access that account via multiple routes; the gmail website, the Gmail app on your phone and tablet, and regular email clients such as Thunderbird. To most users this is seen as an advantage, since wherever you are, whatever tool you're using, you see the same messages. There's no issue such as that one crucial message which is stored on the computer at home which you can't get at right now via your phone.

So it's not "duplication" but quite the opposite. One copy held on an email server and accessible via many different routes.

If you don't like Gmail being involved, choose another provider. But so long as you're using an account which supports IMAP, you'll get this same presentation of your message appearing on all of any of your IMAP-connected email tools.

If you don't want a email provider to store messages for you, set up your account using POP instead, and set Thunderbird to download and delete messages. Then think hard about doing a regular backup, because that copy on your computer is now the only one you have, and when your HDD goes down, it's all gone.

Chrome is a browser. Any other browser (e.g. Firefox, Edge, IE, Safari) will do the same, since a browser looks at the Gmail website.

Thank you Zenos, but I have Carbonite for backing up and some other advantages. I have no need for duplicate messages which crowd the screen unnecessarily (for me).

I changed from Firefox to Chrome in case it would eliminate those tables on forwards. I guess the joke's on me since I still have those tables on the forwards and now I have this additional obstacle. But thanks again. And I wish you and yours a Happy New Year!

P.S. I remember to have read something about having to do with "All Mails". Is there a way to eliminate this folder or button?

It would seem you have a gmail imap mail account.

Gmail keeps all emails in 'All Mail' and labels allow those emails to be displayed in various 'folders'.

If you can see 'All Mail' folder in Thunderbird imap gmail account, you can choose not to see it by unsubscribing from seeing All Mail.

Right click on imap gmail account in Folder pane and select 'Subscribe' to see the list.

Hello Toad-Hall and thank you so much for this ray of hope! I had searched all over for this "All Mail", even signed in to my dormant Chrome as someone suggested, and couldn't find it but now...I realize I don't have IMAP, I created both my Thunderbird and Gmail accounts only with POP. So then I wonder why I got this and how to delete it?

Also, I don't know whether my RCN server allows other than POP.... BTW, I never experienced this until just a few weeks ago, so apparently even not having IMAP it's happening.

What do you think? Thanks again! And Happy New Year!  :o)

Just a note that I found out in the internet how to get to All Mail so I went and hid it so....no more duplicates on my Gmail account! BUT...now it is Facebook that's slapping duplicates on my Gmail account.

I'm telling you, there's NO end to problems in computerland! lol!