In Drupal 7, using Flags (and possibly Rules or another contrib module) how can I force a flag to be unique for a specific content type. I do not want to force users to manually 'unflag' content before 'flagging' a new node. I would like reviously flagged nodes to be automatically unflagged once a new flag has been set.
The goal is to allow users to select a 'default node' by use of flags. Only one default node should be allowed.
How can I force and automate flag uniqueness?