External URLs are a good way to identify potential duplicates.  For example:

1. Adding a release - for example, VGMdb by default presents Latin script text.  There is a user option to prefer Japanese text, but people who don't register would never know this.  MusicBrainz search systems are not good at unifying Latin script and Japanese script data across different objects.  If the user (or the import script) adds the VGMdb URL to the release editor, it should lookup that URL to see if it already exists in MusicBrainz, and warn the user that they may be creating a duplicate of that preexisting release.
2. Adding a work - for example, OCRemix is an English website.  For the same reasons as above, Latin script and Japanese script names differ and will not be easily found.  If the user adds the OCRemix URL to the work editor, they should be presented with any preexisting Work(s) with the same URL.