If a Learner needs to retake a course while still retaining their previous course history, you can archive their course progress.
To begin, select a course then on the Learners tab, select Action -> View Course Progress.
From the Course Status dropdown menu, select Archived.
The learner will now be reenrolled into the course with course progress reset.
The previous course status will be changed to Archived and previous information will still be intact (start date, completion date, etc).