I think the best answer is: It depends on the browser, especially after a post/postback.
Older browsers used to pop up a confirmation dialog to the effect of "the page contains POST data which will be resubmitted", and you could either proceed (resubmit) or cancel out. Since everything that happens in ASP.NET WebForms is part of the FORM element (ViewState, events, etc.), this would cause the entire lifecycle to be repeated.
Of course, this caused no end of trouble with duplicate submissions, so many sites had to come up with workarounds for the dupe problem, and today most browsers just fetch the page from cache instead.
...That is unless you override the cache-control headers and force the browser not to store the page in cache. Obviously, in that case, it can't be retrieved from cache, so it will usually end up being resubmitted. But, again, it depends on the browser - for example, some browsers won't allow the resubmission over SSL, so if that's the protocol in use then the user will just see a message saying that the page has expired / can't be shown.
Come to think of it, probably an even better answer is: As a site designer, you really can't depend on any specific behaviour from the user's browser when the Back button is clicked. If a duplicate submission could have negative side-effects (such as charging a credit card twice), then you need to take adequate measures to prevent that from happening. It's good practice anyway as it's entirely possible for a user to simply double-click the "submit" button by accident.
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