You could define a custom model field derived from models.CharField.
This field could check for duplicate values, ignoring the case.
Custom fields documentation is here http://docs.djangoproject.com/en/dev/howto/custom-model-fields/
Look at http://code.djangoproject.com/browser/django/trunk/django/db/models/fields/files.py for an example of how to create a custom field by subclassing an existing field.
You could use the citext module of PostgreSQL https://www.postgresql.org/docs/current/static/citext.html
If you use this module, the the custom field could define “db_type” as CITEXT for PostgreSQL databases.
This would lead to case insensitive comparison for unique values in the custom field.