This seems to work nicely:

```python
class UserTokenViewSet(PermissionsViewSetMixin, ModelViewSet):
    model = UserOAuthToken

    def pre_save(self, instance):
        UserOAuthToken.objects.filter(
            user=instance.user, identifier=instance.identifier
        ).delete()
```

...deleting previous tokens prior to saving.

https://github.com/tomchristie/django-rest-framework/blob/master/rest_framework/mixins.py#L49