What does this error mean: The model User has two manually-defined m2m relations ...

Question

I'm running into this error when I attempt to syncdb:
auth.user: The model User has two manually-defined m2m relations through the model FavoriteQuestion, which is not permitted. Please consider using an extra field on your intermediary model instead.

I'm really don't understand what it means because I only see 1 model-to-model relations in the model FavoriteQuestion.

class FavoriteQuestion(models.Model):
    """A favorite Question of a User."""
    question      = models.ForeignKey(Question)
    user          = models.ForeignKey(User, related_name='user_favorite_questions')
    added_at = models.DateTimeField(default=datetime.datetime.now)
    class Meta:
        db_table = u'favorite_question'
    def __unicode__(self):
        return '[%s] favorited at %s' %(self.user, self.added_at)

I'm not sure if this is important, but this is also in the models.py file.

User.add_to_class('favorite_questions',
              models.ManyToManyField(Question, through=FavoriteQuestion,
                                     related_name='favorited_by'))

edit added Question model

Django 1.1.1

class Question(models.Model):
    title    = models.CharField(max_length=300)
    author   = models.ForeignKey(User, related_name='questions')
    added_at = models.DateTimeField(default=datetime.datetime.now)
    tags     = models.ManyToManyField(Tag, related_name='questions')
    # Status
    wiki            = models.BooleanField(default=False)
    wikified_at     = models.DateTimeField(null=True, blank=True)
    answer_accepted = models.BooleanField(default=False)
    closed          = models.BooleanField(default=False)
    closed_by       = models.ForeignKey(User, null=True, blank=True, related_name='closed_questions')
    closed_at       = models.DateTimeField(null=True, blank=True)
    close_reason    = models.SmallIntegerField(choices=CLOSE_REASONS, null=True, blank=True)
    deleted         = models.BooleanField(default=False)
    deleted_at      = models.DateTimeField(null=True, blank=True)
    deleted_by      = models.ForeignKey(User, null=True, blank=True, related_name='deleted_questions')
    locked          = models.BooleanField(default=False)
    locked_by       = models.ForeignKey(User, null=True, blank=True, related_name='locked_questions')
    locked_at       = models.DateTimeField(null=True, blank=True)
    # Denormalised data
    score                = models.IntegerField(default=0)
    vote_up_count        = models.IntegerField(default=0)
    vote_down_count      = models.IntegerField(default=0)
    answer_count         = models.PositiveIntegerField(default=0)
    comment_count        = models.PositiveIntegerField(default=0)
    view_count           = models.PositiveIntegerField(default=0)
    offensive_flag_count = models.SmallIntegerField(default=0)
    favourite_count      = models.PositiveIntegerField(default=0)
    last_edited_at       = models.DateTimeField(null=True, blank=True)
    last_edited_by       = models.ForeignKey(User, null=True, blank=True, related_name='last_edited_questions')
    last_activity_at     = models.DateTimeField(default=datetime.datetime.now)
    last_activity_by     = models.ForeignKey(User, related_name='last_active_in_questions')
    tagnames             = models.CharField(max_length=125)
    summary              = models.CharField(max_length=180)
    html                 = models.TextField()
    comments             = generic.GenericRelation(Comment)
    votes                = generic.GenericRelation(Vote)
    flagged_items        = generic.GenericRelation(FlaggedItem)
    email_feeds          = generic.GenericRelation(EmailFeed)

    objects = QuestionManager()

Answer 1

Your code still validates fine. Do you have any other models with links to User?

Answer 2

Looks like that add_to_class statement in models.py file executes twice, put a print there and see what happens