Before a user post a new thread, they can type in the title of the thread and click the button. This will basically automatically open a new window, take the title of thread and place it in the "keywords" section of the search page, run its query, and give you the results. You could even add a little drop down option next to the "Check If Already Posted" button to give the user options on how the want to search (3rd pic).

Hopefully this will cut down on some of the reposts and encourage them to search before posting a new thread. Anyway, it's late and it seemed like a good idea at the time.