When 2 or more samples are assigned to same pad the MPC does not always is able to trigger them at the exact same moment, there is a small delay prob less than 15 miliseconds. two = sounds with a very short delay cause Phase / Flange. (It pisses me off that a machine like a MPC2500 wich cost me a lot a money cant do a simple thing like this...)
I dont see the point of having the same sound with the same parameters twice, it just doubles the volume; but if you really want that, maybe with diferent decay/atack or whaever.. the solution is to tune one of the samples a little bit, if they are not = they dont flange. Although they dont flange the sound might not be very consitent in all pads hits, try for yourself..
This has been covered before. Check this thread:
viewtopic.php?f=29&t=120537