By default, candidate can only take the test once.
If you have allowed more than one social login, it is technically possible to take the test again, using different login.
If you would like to allow a candidate to retake the test, you can delete the candidate and they will be able to retake the test. To do so open your test, navigate to Candidates, click open the candidate and use the Delete option under their name.
Note that only Admins and Owners of the workspace are able to delete candidates. Manager can't delete candidates.