This sentence appeared twice in a review session just now:
I can’t be totally sure, but I think they were both ghosts (I have reviews ordered in “ascending” order, and the second one was definitely a ghost!)
By the way, one case I can think of where vocab might be repeated twice legitimately is when getting a low-level card wrong (I mean “Beginner 1” or “Ghost 1”) then further reviews being loaded into the session when you hit the next hour, so the same card might then get loaded back into the session