This is a duplicate of https://usestrict.net/forums/topic/can-remove-additional-codes-text-mail/. The problem is the same. There’s 2 times because one is supposed to be HTML and the other Plain text. The Content-type: multipart/alternative header was messed up and therefore it didn’t build the message correctly (the mail client shows the one it’s configured to show instead of showing both).
Once you update to bbPress Notify (No-Spam) version 1.14.2, the problem will be resolved.