[Answer]-Django 1.5 Custom User for m2m field 'groups' clashes

Look at the docs for abstract base classes

If you are using the related_name attribute on a ForeignKey or
ManyToManyField, you must always specify a unique reverse name for the
field. This would normally cause a problem in abstract base classes,
since the fields on this class are included into each of the child
classes, with exactly the same values for the attributes (including
related_name) each time.

And look at PermissionMixin code.

One way that I see is to replace PermissionMixin with your own class but it may break a lot more because related names will change.