Google says:
"Each click on an AdWords ad is examined by our system. Google looks at numerous data points for each click, including the IP address, the time of the click, any duplicate clicks, and various other click patterns. Our system then analyzes these factors to try to isolate and filter out potentially invalid clicks before they ever reach your account reports. This detection and filtering occurs over a number of levels including the following:
Real-time systems filter out activity fitting a profile of invalid behavior (such as excessively repetitive clicks)
Clicks and impressions from known sources of invalid activity are automatically discarded"