Yeah, duplicate reports are generally marked as bad. I know, I've recently picked some up myself . It's not a big deal, it happens from time to time. Unless, you're using an addon to keep track, you probably will make that mistake quite a few times.

Some might ask why are you even allowed to report the same post multiple times, but it has it's uses sometimes. For example, when you've discovered new information, and basically want to update it. You might still get a bad report for that, but at the end of the day that's a drop in the ocean.