You can retake a course if it has been updated after you have completed it. To retake a course you will have to reset it. Resetting a course will not delete any information, it will only allow you to retake it.
Compare the dates on the course selection menu
In this example you may NOT retake the Basis of Assets course because the course was completed on 1/6/2015 and the course was last updated on 1/5/2015.
When the Last Updated date is subsequent to the Completed date you may retake the course.
If you try to retake the course before it is updated, the following screen will appear.