As said above, it really depends on your roster and how many good duelists you can field. Once you've been playing for awhile and have a wide variety of units that can fill the same role, even having dupes of amazing units loses its charm. Typically I value diversity more than anything else, because the game rarely gives us maps where two different lanes of the same map are most optimally handled the same way.
In fact, the only unit I can think of that I ever wished I had a half dozen of to field at once is Lisse Lotte. <_< But she's a bit of a special case since she's a ramp unit.