Comment out duplicate 'echo $OUTPUT->header();' line
This is only necessary because another echo $OUTPUT->header(); was introduced further up the page by Sam Marshall's backup/restore improvements commit (https://github.com/sammarshallou/moodle/commit/6f1f72419f1f72cc0416ee9eafce1d1a868c37f2), so this will need to be reverted if Sam's commit is subsequently reverted.