a small warning is shows saying that you have already reported that post. This warning could also contain some information like the time of the report or even the reported text.
I'm not asking for something too flashy either, a simple red text similar to the "someone has posted while you were typing" text that is usually displayed.
If you really care about that , when you reporting a lot of posts in a thread and you report a few twice then report it slowly and concentrated.
The computer is only as good as the user sitting in front of it , about the warning i guess this would be a problem when other Users report it then.
Dont care about the bad reports , keep reporting and at some stage you will find your way that this not happen again or often.