Comment by nkurz

Possibly you could split submission into multiple steps? The user first submits just the URL. This returns the results of the dupe detector (if any). They then confirm submission (or resubmission) in the next step.

The first submission could also test the URL, and pre-fill the title field with the actual title of the page. The user then edits the title (if desired), and confirms to submit the page. Or bails out when they realize the submission already exists under a duplicate URL.

This could be done within a page with AJAX, or kept as it is with multiple pages. In either case, you'd leave the final choice of title and URL with the user, and are only offering them more information (and an easy option to cancel) during the process.