Django - UserProfile m2m field in admin - error

My models:

class UserProfile(models.Model):
    TYPES_CHOICES = (
        (0, _(u'teacher')),
        (1, _(u'student')),
    )
    user = models.ForeignKey(User, unique=True)
    type = models.SmallIntegerField(default=0, choices=TYPES_CHOICES, db_index=True)
    cities = models.ManyToManyField(City)
class City(models.Model):
    name = models.CharField(max_length=50)
    slug = models.SlugField(max_length=50)

In admin.py:

admin.site.unregister(User) 
class UserProfileInline(admin.StackedInline):
    model = UserProfile

class UserProfileAdmin(UserAdmin):
    inlines = [UserProfileInline]

admin.site.register(User, UserProfileAdmin)

@receiver(post_save, sender=User)
def create_profile(sender, instance, created, **kwargs):
    """Create a matching profile whenever a user object is created."""
    if created:
        profile, new = UserProfile.objects.get_or_create(user=instance)

But when I add new user and select a city I get that error: IntegrityError at /admin/auth/user/add/ (1062, "Duplicate entry '3' for key 'user_id'")

What is wrong with my code? If I don't select any city - user is added properly. Some way, user is being added to UserProfile more than once.