If someone has posted in the discussion with a message which contains spoilers and then you open that discussion, if they edit their reply and then you post you will see duplicated spoilers at the end of the post (very likely for the same reason that the post author would see duplicates after they edit).
This is hard to reproduce, but it is reproducible. It's also fairly outlandish situation which probably wouldn't happen very often.
This is NOT the "I edit my post and multiple spoilers appear" bug.
It is caused by the same issue, though, it just confirms it affects other users and not just yourself!