Ignore multiple matches for a user-customcert combo in the customcert_issues table.
In our case, the scheduled task died when:
$issueid = $DB->get_field('customcert_issues', 'id', array('userid' => $enroluser->id, 'customcertid' => $customcert->id));
returned 2 rows, which shouldn't happen, but did anyway.
This means in rare circumstances (i.e. the race condition) the user may get multiple certificates (e.g. one via the UI, one by email) but that is better than not getting one at all and the whole job dying.
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