Double post error

Got this when trying to reply/post some pics in a Nosebleed thread

Don't double post (too lazy to automerge)

Bug?

Nope, its absolutly normal, no worries here.
Its just the site's script telling you that you can't double post.

Thats why if the last post in a thread is yours, you'll have to edit the old post, not make a new one. Most likely to prevent spam.