AllThingsD's Walt Mossberg says Apple will restore iTunes duplicate detection in a "minor update." The pre-iTunes 11 duplicate detection feature lets users easily identify items that occurred in an iTunes library more than once. Having done that, deciding which ones to keep and which to ditch was easy enough.
We've speculated that duplicate detection was interfering with iTunes Match, but that's just a guess. It's good to hear that Apple's working on restoring this functionality.