Stop students from testing twice

QUESTION
I have been using thatquiz for a few months now, and normally when my
students take a test, they cannot go back in once they have seen their
score. I had one boy trying to retake his test that I already had a
score for! He is sneaky about a lot of things, so I just happened to
be keeping my eye on him -- thank goodness...or I might never have
caught it. Do you have any way of letting me know which ones took the
test twice?

This has not happened to me before. What did I do wrong? How can I
make sure this does not happen in the future?

ANSWER
 Take a deep breath.
 If a student manages to take the same test more than once, the first
grade is the only grade which is saved.
 Even if the student's name doesn't immediately disappear from the
list, future attempts on the same quiz will not be registered unless
you, the teacher, manually remove the first grade saved from the
grades table.
 If a student finishes a test, and the grade is sent, thatquiz.org
always checks if there is an existing grade for that student on that
exam, and if there is already an existing grade, the newer grade is
simply discarded.
 So... if a student thinks he/she is being clever by taking the test
again -- let them!  They'll get some extra practice, a chance to
correct their mistakes, but it won't change the result you have in
your grades table.
 If you are still worried about this, you can click on the test name
to change its configuration.  There are some options which may be of
interest:
1. Show number complete when testing (students won't know how many
they're getting right/wrong as they test)
2. Show grade alone when finished (students won't know which answers
they got right/wrong even when they finish)
3. Do not show results when finished (students won't even know what
score they received).