Well, for one there is say... me, who does not use dupes. It's rather simple though. It's because i have set a rule that there exists no same 2 characters in my "universe." Also, for future maps like Majin, you cannot use dupes anyways so might as well adhere to that already. I see no need for dupes since i have a good diversity of units to cover most things.

The one thing i would do is to raise several Aishas for the hilariousness of metor storming maps, but that is the one exception.

As for the Grace specific one, i would not raise a 2nd Grace. Sure she makes for a good tank and deals good damage, but she also falls behind many others and costs a lot to put out. I see no reason to raise a second one to have as a "tank/dps" since her cost is high and she only deals good damage on skill compared to other units that are ranged or high defense without the need of a skill to make it so.