Python random character string repeated 7/2000 records

I am using the below to generate a random set of characters and numbers:

tag = ''.join([random.choice(string.ascii_letters + string.digits) for n in xrange(36)])

I thought that this was a decent method. 36 character length, with each character being one of 36 unique options. Should be a good amount of randomness, right?

Then, I was running a query off an instance with what I thought was a unique tag. Turns out, there were SEVEN (7) records with the same "random" tag. So, I opened the DB, and ran a query to see the repeatability of my tags.

Turns out that not only does mine show up 7 times, but there are a number of tags that repeatedly appear over and over again. With approximately 2000 rows, it clearly should not be happening.

Two questions:

(1) What is wrong with my approach, and why would it be repeating the same tag so often?

(2) What would be a better approach to get unique tags for each record?

Here is the code I am using to save this to the DB. While it is written in Django, clearly this is not a django related question.

class Note(models.Model):
    ...
    def save(self, *args, **kwargs):
        import random
        import string
        self.tag = ''.join([random.choice(string.ascii_letters + string.digits) for n in xrange(36)])
        super(Note, self).save(*args, **kwargs)