While investigating a client's concerns about click-fraud on their AdWords campaigns today, I came across an interesting phenomenon. The good news was that there wasn't any readily apparent fraud, at least in the sense of large click counts being traced to one IP address. There were, on the other hand, a number of cases of individual IP addresses hitting the same ad 2-3 times.

Initially, I assumed this was just a matter of double-clicking or user impatience, but the timestamps didn't bear that out (and, I assume, Google anti-fraud algos account for this); most of the clicks were at least two minutes apart, and some were much longer. My suspicion is that one of two things is going on:

1. Users are hitting an ad, visiting the site, and then hitting the ad again later through the back button
2. Users are treating the ad like a bookmark and are using it to re-access the site. 

On the usability side, I see behavior similar to (2) all the time; it's amazing how many people visit a site through a link to a referring site, by repeating a previous search, or by typing the URL into the Google search box.

The real downside of this behavior is that it creates click inflation, a cost margin that's effectively being added to our entire campaign. In my brief and very unscientific experiment, a full 15% of AdWords clicks were non-fraudulent duplicates. That means that my client effectively paid an 18% markup on today's visitors, a pretty steep tax on clicks that are already costing almost $1 each. Has anyone else noticed this trend and have you found similar numbers?