I can’t say I would never, ever use the same name as one that had already been used in my family or my DH’s family, but I would very much hesitate before I did so. If it were the name of a sibling’s child or a cousin’s child (and the cousin was someone I had contact with, even if just a little bit), I really don’t think I would use the same name. If it were a cousin who had chosen the name, and I had never met the cousin (as is the case with some of my DH’s cousins because he has such a large family), then maybe I would consider using the same name, but I’m still not sure I would do it. I would be especially hesitant to choose a name already in use in the family if my child were to have the same last name as the other child who already had the name. That would just be very confusing for future genealogists! I’m lucky, however, in that I have 12 nephews and nieces (on DH’s side), but I’m not the least bit tempted by any of their names. My sis doesn’t have kids yet, and neither do I, and at one point, when we were discussing names, it looked like we might have some of the same names on our list. But since then, I’ve grown more interested in other names. So I doubt there will be a problem. Anyway, if she uses a name before I do, I will just have to find another one. My cousin has some cute names for her kids, but, again, I can live without those names being on my list. There are, after all, a lot of names out there.
There are exceptions, though. The main one being family names from shared direct ancestors. Anyone in the family should be able to choose a family name, even if it’s already “taken”. There are probably other exceptions, too.