Possible way to catch dupes on user entry: When a user begins submitting a URI we recognize as similar to one already entered, automatically ask "This might be

Possible way to catch dupes on user entry: When a user begins submitting a URI we recognize as similar to one already entered, automatically ask "This might be a dupe. Sure to commit?" and list the similar entries we already know of. Similar to the "this username is already taken" autocomplete box on many sign-up forms.