But one question: How do I mark those 'old' issue #'s as duplicate? I don't see that as a Status option. I can mark the newest ones Rejected and say 'Duplicate of xxx' in the Notes field.....?

Well, you can do that, but the better way is to click "add" under "related tasks" and then choose "duplicates" and put the issue number that you want to keep. Then the system maintains those links so that if any of them have useful information, it's easier to find a path between the three of them.

--Dan