verify uniqueness of names

[graybeal]

Steve Emmerson remarked, in the context of an issue with dram:

1. Each name should unambiguously identify a unit;
2. Each symbol should unambiguously identify a unit;
3. Long names (e.g., "avoirdupois_dram") should be supported.

I think there's at least one case where the same name is used in two different places, one of them in the derived vocabulary and one elsewhere. I'm not sure how we dealt with it; this is a reminder to me to find out and confirm it makes sense.