Stop repeat threads appearing

[simalert]

We have an area of our forum for members to list certain free offers when they appear on the net. It's a great part of the forum, but the only downside is that a lot of members tend to post the same offer because they either don't see it's already posted or they just want the 'thanks' received for posting an offer.

I need an idea to stop people repeat posting, whether it's an automated search that recognises repeat posts or just a warning message that appears as a popup in this area of the forum only to warn people to search before posting.

Any ideas anyone?

Thanks