actually you cannot simply enrol and re-enrol someone if you need them to retake the course. Their completion data is tied to the enrolment so if you unenrol them, the completion data goes with it. They can no longer see it on their transcript. That said, if you do re-enrol them, they will appear to be complete but the completion date will change to the date you're-enrolled them.

If you’re needing to have your users do a course annually, you will need to create an annual version of your courses…

We are expecting to see a major enhancement later this year that will allow you to ‘archive’ previous enrolments which should solve your issue but we dont know when yet…