Is a dll shared variable equivalent in behavior to a session variable?

I'm trying to keep the user from submitting a form twice by accident (by double clicking on a submit button).  To do that, I have a class in the 'app_code' folder that records the time that the button was clicked.  
My question is this - would it be better to store the time in a session variable?  Or is having a "shared" (this is a vb term) public variable in a class in the app_code folder better?  Does the app_class have one copy of that variable for every user?  And for each user, does it 'remember' the value of that variable across postings of pages?
Thanks,
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