How can I tell if a somebody used the login 'Accounts' today to log into a pc in my Active Directory domain? It seems to be causing errors if 2 people are logged in using the same username and I don't know if this is the case or not.
To see the last time a particular account has logged in to your domain, you can run the net user command via a command prompt. Syntax is as follows (without the brackets):
That will return several pieces of information, including Last logon time.
With Active Directory a user can be logged into different workstations without causing problems. What are the issues you are seeing? If you have the proper auditing on your DCs you could search through the event logs for login events for that user.
A more accurate answer could be given if you provide more information on what OS you are running on your DCs and what Domain & Forest Function level you are running.
You could also use the Lockout Tool to search the username if your problem is related to the account getting locked out. It will search the event logs for bad logon attempts on the DCs and show you the most recent attempt.