Which layout do you use? Does it contain more than one ad?
Most likely you had one visitor who right-clicked on two ads.
While the reason right above seems a good possibility, I'd just chalk it up to strange reporting issues. There is no real formula for what will show up first and what will be correct the next day.
I could see right clicking and opening in a new window as totally plausible. I know I will open multiple ads if they strike me as being interesting or useful.
"Which layout do you use? Does it contain more than one ad?"
I use a variety of layouts, mostly the leaderboard or the 160 skyscraper. That was a one-time event, so I chalk it up to either a quirk in the reporting or the outside possibility of someone right-clicking to two ads.
what is all this "right clicking" jarble about?
If you click and ad, and use the back button, and click another ad, thats 1 impression and 2 clicks. You are not reloading the page, so it is 1 impression, but 2 clicks are occuring.
This happens all the time
you actually ARE reloading the page in back-button push, as the adsense code is an external include, right?
Or am I wrong?
would put all my adsense earnings on it just being a reporting hiccup and nothing else.
No, it won't reload.
I don't think it would
If it does, possible reasons will be, Shift + Click or rightclick as others suggested.
In my experience the ads typically do not re-load when you go back. So both the back-arrow and the new-window explanations would work.
This might also happen if the visitor clicked on the same ad twice. Lets say the visitor clicks on an appealing ad, but the advertiser's page never loads. So they click on the ad again. Maybe the advertiser's page loads this second time, but maybe not.
The big question is whether the AdSense publisher gets credit for either one click or no clicks.