I've just been banging my head off the desk with this problem but have finally (I think) found a solution - it looks like they've included the option but changing Multiple Submit but have then disabled it as it's not supported or something. You can't change it through the Moodle interface, you'll have to make a change directly on your underlying database to make this work:
UPDATE `moodle`.`mdl_feedback` SET `multiple_submit` = '1',
Replace 'moodle' with the name of your database and 'mdl' with prefix used for your database tables.
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