How to detect a session variable has changed?

I have a VB.NET web application that stores a database KEY (let's say it is an Employee ID) in a session variable (i.e., SESSION("KEY") = employeeID) and then uses that session variable in all it's other pages to ensure that the proper record is displayed and updated.

However, a problem arises if I open two Internet Explorer windows and point them both to the same web app:
- window 1: select record 11 then go to the edit page in the same web app
- window 2: select record 22 then go to the edit page in the same web app

When window 1 clicks the save button, the SESSION("KEY") has been changed and therefore the data for 11 is actually saved as 22.

Any suggestions on how to prevent this data corruption?