The procedure is quite lengthy
1,you should refrence designators in your SCH like, C21_1 and net names should also have "_1" in there end.
once you will have refdesg and netnames like these when you will copy the refdesg will automaticly increses and net names will need to replace from _1 to _2 and so on.
2, once you copy to the required quantity generate and load the netlist in Allegro PCB editor.
3, now go to create module and make a module of your current PCB and open that module in new canves then go to export placement.
4,open the generated placement file in notepad and replace "_1" to "_2" and save the file
5,now go to import palcement and place the components at new palce and afetr that copy all clines and shapes ect from initial PCB to 2nd one.
6, repeat point 4 and 5 as many times as needed.