The Markbook for a class will always display students' most recent attempts at a test. If your student retakes the test, then their last mark recorded will be replaced by the newer mark (better or worse).
To view their attempts, you can review results within the test itself. You can do this by selecting the test you want to view, and clicking "View next attempts at test" to view results from any students who have taken the test again.
You can also monitor their progress within the topic tested through Track Progress.
For more information on how to do this, please see the article below: