Hmm.  But if there is one spot that generates the same warning (with the same caller) over and over again (e.g. a shared example group that's used many times), it could get really spammy, where the user is getting hundreds of messages.  That seems bad.

If you can find a way to use a `Set` w/o causing this side effect, then sure, go for it -- I'm a fan of using `Set` when the semantics of the collection call for it :).

Actually...maybe we don't want a `Set`.  Consider that `Set` is not ordered, and as a user, I think it would be confusing for the deprecation messages to be printed in an order that doesn't match the order the specs were run in.