Question with UserControls and Session Variable

Could someone help:
 
     I have a UserControl that via javascript calls a Popup window.  From that popup window a user can make a selection, then the popup saves that selection to a Session variable for the calling UserControl to process.  That methodology would worked fine if I was calling a single popup from a single UserControl, but I have multiple instances of the UserControl on a page, the Session variable idea fails because each control can call the popup and overwrite the Session var.
 
Would anyone have any ideas how I could save the Popup selection data as unique values for each instance of my UserControl.
 
Thanks
 
Dave.
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