Profile: warn the user when two different fields clobber each other

Use case: a profile form with a required 'Email (Work)' (email-2) and a non-required 'Email (Contact)' (email-Primary), with the default location set to work.

Current behaviour: a new user comes to this profile, puts in the (required) work email, leaves the (non-required) other email empty. Result: as work is the default location, empty email-Primary gets resolved to email-2 and the empty value clobbers the email-2 value provided by the user, resulting in a contact with no email address whatsoever.

Desired behaviour: when an administrator creates a profile, either not allow for the profile creation or at least warn the administrator if this profile contains two fields that in the end resolve to the same field in the database (like email-2 and email-Primary in the above, real-life scenario).

(Tentatively scheduling the fix for 3.2, but feel free to reschedule to a later version based on your opinion on the severance of this issue.)