I made a change, adding a unique constraint to a model, within the abc application and did a
./manage.py schemamigration abc --auto
That created a migration file but as well as the expected change the new migration file also contained a number of add_column statements which are adding columns which were previously added in an earlier migration (and which have been the subject of a migrate)
I'm really puzzled as to why this has happened and what to do about it.
Will the add_column statements just be ignored if I do another migrate ?
OK thanks to the #django-south irc channel I've figured this out.
This type of problem can arise when activity has taken place in different source control branches and, as a result of a merge, the dictionary of frozen models, which appears at the bottom of a south migration file, is missing some stuff which has already taken place. The result of this is that the next schemamigration tries to produce the "missing" changes.
The fix is to manually edit the migration file which was created by the schemamigration before doing migrate. This will get things back into synch.
There's some information about issues in the later part of this section : http://south.readthedocs.org/en/latest/tutorial/part5.html#team-workflow .
Thanks to carljm and maney on #django-south for helping me with this.