Process > Run CAL > Undupe. If they've been quantized 100% to start on the same tick, you can leave the window at 0 (no delta in start times). But if two notes of the same pitch from a live performance are getting quantized to the same time, you probably need to adjust your quantize settings or tweak some individual note positions before quantizing so the one does not move the wrong direction. Normally, you wouldn't want to create dupes and subsequently lose notes in the process of quantizing.