Update Query For Capturing Log Out Time Of User

I have done some searching to find a method for logging & timestamping when users log in and out. I have no problems with capturing the Log IN information my problem lies with the log out portion where I am trying to use a Update query to update when the user logs out of the database:

I can get the update query to do what I want, but for some reason it is creating a duplicate record (exactly the same as the last record updated)

Here is the code I am using:

UPDATE LogInTable SET LogInTable.LogOutTime = Time()
WHERE (((LogInTable.LogOutTime) Is Null) AND ((LogInTable.UserName)=LogUserName()));


Basically I am trying to tell the Query to ONLY update records where the "LogOutTime" is Null AND where the UserName field in the table = the username of the person trying to run the update query.

Please help I can't figure out how to prevent these duplicates. :(