“ASP.NET is preventing the click event being called more than once. Is this true?”
No, actually what is happening to you is that your events are blocked by the lock of the session. (What session is on asp.net, is a module on asp.net that can keeps data for each user that visit the site)
So when you make a submit, and the page is running this submit, and then you make a second one before the first one replay, actually the second one is waiting the first to finish and unlock the session.
To make a test and prove that the session is locking your request, turn off the session and try again your test.
relative:
Web app blocked while processing another web app on sharing same session
What perfmon counters are useful for identifying ASP.NET bottlenecks?
Replacing ASP.Net’s session entirely
Trying to make Web Method Asynchronous