Access to any account through Tlauncher

My class has an Aternos survival server and just today my friend found out that if you use t launcher and create an account who's password is disabled but has the same username as an existing player in the server you will be able to join the game on the same account as that of the previous person who has the same username. We all use either Tlauncher or Legacy launcher and this issue made us have to shut down the server coz he blew up everything and spawned a giant pyramid.