Off topic, I've seen the page that displays what the other ads are paying per click. Does anyone remember how to get to that page? Am I clear? I've looked all over and I know I seen it before, but can't find it now.....
Google clearly says that it attempts to prevent multiple charges for duplicate clicks. It doesn't say how AFAIK. However, I know that I see cookies on my computer related to AdWords ads I've clicked on, so tracking clicks by cookies is obviously one method, very likely the main one.
But Google doesn't require that the user activate cookies. So do you get multicharged for no-cookie users who multiclick? I find no answer, but it they are on the ball, they will assume that no-cookie users multiclick at the same rate as cookie-enabled users, and reduce the count by the same percent.
I suspect that no-cookie multiclick users are very few. Web browsers have cookies enabled by default, so for the most part only sophisticated users will disable them. And sophisticated users are much less likely to multiclick on ads.
Edward
They do not necessarily consider all multiple clicks by the same user as fraud clicks.
I have seen one user click 8, 10, 20, times (I know same user from my own tracking cookie) Google would not refund clicks like that automatically.
Whatever their criteria or process is a secret, as it has to be.
I checked my adwords stats to see if they'd been counted, but fortunately it seemed those 255 clicks had not been counted by AdWords. Wonder if that competitor helped raise my Adrank with his/her mindless fraudulent clicks :)