'Validate that exclude is a sequence without duplicates.'

def validate_exclude(self, cls, model):

    if cls.exclude:
        check_isseq(cls, 'exclude', cls.exclude)
        if (len(cls.exclude) > len(set(cls.exclude))):
            raise ImproperlyConfigured(('There    are    duplicate    field(s)    in    %s.exclude' % cls.__name__))
