Same mail purpose, multiple addresses?

We are beginning to use Mail Purposes in earnest and I am trying to understand where things might go wrong.

What happens if there are two non-primary addresses in the same control group with the same purpose marked?  Put another way, what will FT_GET_ADDRESS or AP_GET_ADDRESS do if they find two addresses which match the purpose?  Will they error out, or will it simply pick one of the addresses randomly and return it.