Dupes aren't as important as you'd think.
1st, 2nd, 4th and 6th stars each increase the stats of a unit by 5%
while, stars 3rd, 5th and 7th upgrade the z ability of a unit.
If you were considering pulling for dupes then it's not recommended because at most you'd be getting only a 20% increase in stats and 10% increase in z ability and this in total would require 16 dupes.
