We have to plugins for this. One is a block that allows users to reset themselves and the other allows the admin to reset based on the number of days it's been since their last completion. We got both of them for free from the Moodle plugins site, although we did have to modify them for our Totara 12 install.
Totara Learn Open Discussions
Complete courses multiple times and keep a record of previous completions
Re: Complete courses multiple times and keep a record of previous completions