(ASK> 1 user can Login twice with same username

Hello everybody..

i had some problem that 1 user can login twice or more with 1 username, just only add space character after type the username (ex: "1000 ") or ("1000 ") or ("1000 ") they can login in my opinion unlimited account. i already search other reference that same problem, but i can’t find it. Maybe you can help me to resolve this problem.
Thank you very much.

anyone can give me some clue or answer? please..