On the Fedex Shipment web application, in fine-print at the bottom of the page:
Please note: Click the Ship button only once. Expect some delay due to transmission time. Do not click Stop or Reload; it may cause a duplicate shipment transaction to occur.
This is frankly inexcusable. If the form doesn’t appear to work, the user will inevitably click on the button more than once. The code should accommodate this behaviour, not weasel out of it with small print. In developing the form there was an important acceptance or test case left out, that for monkey behaviour.