Badge uniqueness - can't issue a badge to the same earner multiple times

[abbycabs]

From the docs, it looks like you should be able to specify whether a badge can be earned more than once by an earner:
https://github.com/mozilla/badgekit-api/blob/master/docs/badges.md

unique  boolean - True if the same earner can only earn the badge once.

But in the actual database, there's a unique key preventing multiple badgeInstance entries with the same earner email and badge id.
https://github.com/mozilla/badgekit-api/blob/master/app/migrations/20140603150606-initial.js#L117