Django引发了MultipleObjectsReturned

Question

我有一个班级：

class RecentCar(models.Model):
    class Meta:
        db_table = 'recent_car'
        unique_together = ('user', 'car')
    user = models.ForeignKey(User, blank=True, null=True, related_name='my_cars')
    car = models.ForeignKey(Car)
    created = models.DateTimeField(auto_now=True)
    is_favorite = models.BooleanField(default=False)

项目适用于使用nginx和flup的VPS。 一个views.py代码：

try:
    recent_car = RecentCar.objects.get(user=request.user, car=car)
    recent_car.is_favorite = True
    recent_car.save()
except RecentCar.DoesNotExist:
    RecentCar.objects.create(user=request.user, car=car, is_favorite=True)

对于1000件物品，我有3-6个错误：

MultipleObjectsReturned: get() returned more than one RecentCar -- it returned 2! Lookup parameters 

为什么会出现这些错误？