Locking out all but one user on an ASPX Page

Guys,

I'm not sure how to ask this or even what Im looking for, so Ill try to explain. Please point me in the right direction:

 I have a web app that accepts appointments. The page that accepts appointments needs to be available for only one person at a time. (User1)

 If someone else(User2,3,4,5, and so on) tries to access the page at any time, it throws an error message, or just a simple message, until User1 goes to another page. At that time, the the user in waiting will automatically be presented that page.

Is that possible in ASP.NET? Im sure it's a server call, but Im not sure wher to look or the correct phrase to look for.

 

Thanks for all of your help!!!!

 
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