Can one use a 15 amp (double pole) outlet on a 20 amp circuit (12-2 wire)which feeds into a 20 Amp single pole. Are the Amps reduced at the end of the circuit because one use the 15 Amp feeder? Thank you.

I'm not sure what your question is but I can tell that you are confused.
Tell us exactly what you want to do and someone will tell you how to do it.

If you are asking whether a duplex receptacle listed for 15 amps can be installed on a 20 amp circuit, then the answer is yes it can.

However the load plugged into that receptacle should not exceed 12 amps.