Have you install adlogger on your sites? It happens to me couples of times before. From what I've seen through adsense overview, only few from the multiple clicks will be accounted.
Yes, contact google first because obviously someone if out to take down your revenue. They will keep doing it.
This happened to me recently on two of my sites, and I just contacted google, and with a pretty quick reply, they worked with me. They were very understanding, and disregarded those clicks. But i would definitly contact them, just to be safe.
dont worry. just contact google and relax. Then nothing happen to your account unless you bombed click your site