also reported at: https://bugs.launchpad.net/ubuntu/+source/evolution/+bug/676390

Created attachment 179812 [details] [review]
evo patch

for evolution;

This is not a full fix for the issue, but it may fix most common scenario, which means opening a new message or reply to a message (the body contains a signature), edit it through an external editor, and when closing the external editor then the text is replaced in the message body and the signature is not duplicated.

Unfortunately the signature will be duplicated when one begins to play with sending account settings or signature setting itself. It's because the relevant information about previous signature position got lost when passing message body to the external editor, so the composer doesn't know where the signature was placed (note of cases like top/bottom posting/signature placing; replying to a message and so on) and cannot replace it there.

Created commit a10247b in evo master (2.91.90+)