Why does my Django 1.3 logging setup cause all messages to be output twice?

My Django 1.3 logging setup causes all log messages to be output twice. I've read that importing settings.py two or more times has caused this problem in the past, but Django 1.3 has new logging features, and I don't think I'm importing settings.py twice anywhere.

settings.py config:

LOGGING = {
    'version': 1,
    'disable_existing_loggers': False,
    'formatters': {
        'simple': {
            'format': '%(levelname)s: %(message)s'
        },
    },
    'handlers': {
        'console': {
            'level': 'DEBUG',
            'class': 'logging.StreamHandler',
            'formatter': 'simple',
        },
    },
    'loggers': {
        'custom': {
            'handlers': ['console'],
            'level': 'DEBUG',
        },
    }
}

Code:

import logging
log = logging.getLogger('custom')

log.debug('message1')
log.debug('message2')

Output:

DEBUG: message1
DEBUG:custom:message1

Thanks for your help.