Multiple postbacks of ASPX page

I have an aspx page with a simple form to send emails to pre-defined lists of users. On the longer lists the page usually times out before the emails finish sending but this开发者_如何转开发 has never been an issue.

Today something weird happened and each user got four emails. In the log I could see three new threads crank up one at a time and start over sending from the beginning of the list.

Any ideas? I absolutely know I didn't intentionally refresh the Web page myself, and certainly not three times. But could the browser (IE8) have done it? Would it post again trying to re-establish a connection when it timed out? Or when I switched back to the browser window from another app? I have never seen behavior like this before.

First question would be whether there is any reason to do a long-running task syncronously, i.e. lock up a thread that should be serving web requests for something that could be done in the background, while the browser sits and waits for a response that its probably not going to get. I'd look into running this asynchronously unless there's a very deliberate reason not to.

Secondly have you looked into creating some kind of locking mechanism such that the process can't be started more than once? I have processes where I add a token to the application cache (and remove it when I'm done) so that if the token exists the process won't run again (the call to the asynch task isn't made), and that does the job. That way it doesn't matter how many clients call your code, you prevent things happening more than they should.