I have a relatively simple Django app, with quite heavy usage that is responsible for quite some concurrency in the db operations. 

I have a model Post with a m2m to a Tag model.

A single line in my code, `p.add(t)` is repeatedly causing mysql exceptions (where `p` is a Post instance and `t` is a Tag instance.)

```
IntegrityError: (1062, "Duplicate entry '329051-1827414' for key 'post_id'")

```

When this is raised I can manually run this `p.add(t)` successfully, so it must have to do with some peculiar state that the db/app are in at the time of normal execution. It happens about once every 1000 tag-adding attempts, without any pattern that I can detect (i.e both numbers in the "329051-1827414" pair of the example change)

A `CHECK TABLE` in mysql on the relevant table shows that they are all seemingly OK.

Any ideas?