Cannot select new user for test drive event registration

When doing test-driving the online event registration, the first page has a user preset. This user can be changed for testing with a new user.
Initially my user I have logged me in is preset. This user registers and the participant record is written.
Trying it a second time will display an error, because the program checks for the the email-address not yet registered. It seems that it either takes the email of the user that is logged in or the one of the privious registration. In effect no more than one user can be tested.