[DigiTechSupt]

So you're saying you do NOT have any MIDI cables going back to the MIDI in on the MPC?

I think, given your signal chain, that the MPC is double triggering - when you play a note on the ProKeys, it's re-triggering the MPC, causing double notes. You have to set the MPC so that it does not respond and only passes through the MIDI data coming from the ProKeys.

Either that or get an additional MIDI interface so that you have more ports to connect the ProKeys to, rather than running it through the MPC...