The problem with duplicated remote comments seems to be gone (I am writing this comment from my own server).
However, I discovered another issue with remote comments like this one: https://socialhub.activitypub.rocks/t/1b12-vs-guppe-groups/5032/13.
Its ActivityPub ID is
https://socialhub.activitypub.rocks/ap/object/07746fd71af9af4f0789f9e4798f33ec
and its attributedTo property has the value of
https://oisaur.com/users/renchap
meaning "actor from oisaur.com created object on socialhub.activitypub.rocks", which generally shouldn't be allowed (unless this actor is explicitly authorized to do so).
I think Discourse should show the original ActivityPub ID of the remote comment when user clicks on the "ActivityPub" button. This probably affects federated activities as well.