A suggestion...

[bossedit8]

I don't think so. So far, if you are a mod you will see that that post has been reported before you take action to it but for a regular user it's not the actual case until you hit the report button.

[Mable]

prophpbb has this feature. But i dunno about whatever Joey uses if it would work phpBB or so.

[underFlo]

Afaik if you try reporting a post that's already been reported you get a message that It's been reported already anyways.

[Choco]

If you try to report a post that has already been reported, it will give you this;

"This post has already been reported."