Is it possible to prevent one account from being singed in at the same time from two different places/devices.

One example for this is to prevent two people sharing an account at the same time.

This should prevent simultaneous login:

ACP -> Advanced Configuration -> Server Environment -> enable "Check IP address when validating session?"

(But please read the text below that setting; if you are on an intranet that may cause login problems for everyone.)

@Tom S., check this mod too: