Validate new date format string submission.
function system_add_date_formats_form_validate($form, &$form_state) {
$formats = system_get_date_formats('custom');
$format = trim($form_state['values']['date_format']);
if (!empty($formats) && in_array($format, array_keys($formats)) && (!isset($form_state['values']['dfid']) || $form_state['values']['dfid'] != $formats[$format]['dfid'])) {
form_set_error('date_format', t('This format already exists. Enter a unique format string.'));
}
}