Can a form execution occur twice?
===

I've obviously missed something somewhere.

In some way a customer has managed to do a booking twice within a 11 seconds delay (checked time). My first though was that the user might have done the booking, then backed in the browser and done the booking again. However this is not possible since I had at least 2 validations which I know would prevent this behavior.

The only conclusion left for me is if the customer tried to submit a booking, our webservice somehow froze in maybe 11 seconds, the customer aborted the request and submited again, therefor allowing these 2 bookings to happend at the same moment.

Apart from that, I got no idea what could cause it. Delay in MySQL insert perhaps?

So basicly what I wonder mostly is;  
**What can cause a form to get submited and executed twice, with the second one bypassing all required validation?**