They shouldn't be able to.
The registration process will tell them that email address is already in use and they need to choose another - "
[email protected] is in use".
Also if they attempt to register with your name they will be shown "That username is already in use or does not meet the administrator's standards. If you are ABCDEFG and you have forgotten your password, click here."
If you suspect that your account has been compromised PM me with the details.