This has happened to me a few times. I am looking at the review queue and see a question asked 2 hours ago, which I am certain I have seen a few days ago. I then open a new tab in my browser find the question I saw a few days ago. I then go back to the review and mark the question as a duplicate.
Then I am getting the error message "Questions cannot be duplicates of themselves". But the new question was asked 2 hours ago, and I am marking it as a duplicate of a month old question asked by a different user.
If I think this is a bug in the review process, would that be a correct conclusion?
I have seen this again. I was asked to review a question posted two hours ago by user2282900. I tried to mark it as a duplicate of a question posted three weeks ago by Mike Furlender. Again I was told
Questions cannot be duplicates of themselves.