Sorry for my english.
This is my idea but I don't if it is applicable.
the user-table have 2 fields : UserID, DateTime1
The client side for suppose every 1 minute update the DateTime1
according to the server computer system time. ( can we get the server side system date/time ? )
If the user attemps to re-log again, server will check the difference between sytem time and DateTime1, if the difference is more than 1 minutes than we can assume that the user session has crashed and is allowed to login in again.