Windows NT Domain Controller

Hi,

Here is a Situaution,
I am using Nt4 Server as PDC… One day my users were unable to login.. I checked the Eventviewer of PDC machine… Its NetLogon Service was stopped and In Event Viewer…
The error was
“THERE IS ANOTHER DOMAIN CONTROLLER RUNNING IN THIS DOMAIN”

Can anybody put some light on it that how is it possible and what can be done to avoid this sort of things as this event gets everybody out of Domain

Immediate help is appriciated

Thanks

Parth