I need to correct my intial post. The duplicate email created by Gmail contains the attachment with the same name as the attachment in the original email, but its size is changed to 1 KB.

I’m afraid this is the way things are implemented at Gmail. Thunderbird actually creates a copy of the message while stripping the attachment(s) and deletes the original message locally, but the original message is not removed on the server.

I think this was reported in this bug, but as this article says, you would still need to delete the original message using webmail in the ALL Mail folder.

Note that this behavior may actually come in quite handy when it comes to saving local (mailbox) disk space and storing the attachment elsewhere or just removing it if no longer needed. As the bug can also been seen as a feature request and considering the amount of work it would take to implement it (if Gmail cooperates), all 3 things add up for low priority.

On webmail, one could forward and strip such messages to themselves. For Thunderbird, both the original message and the stripped copy may be available when setting up new profiles (I think).