A WebmasterWorld thread has almost 50 posts from Google AdSense publishers who have received duplicate 1099 forms. A 1099 form is a IRS form used to report earnings to the US government.
Google has confirmed in the thread that "some publishers received duplicate 1099 forms." But they assure the publishers that their "earnings won't be reported to the IRS twice."
AdSenseAdvisor apologized for stressing some publishers out over this and afford further assistance.
If you find an error in your tax form (wrong name, wrong tax ID, etc.), please don't hesitate to contact us and we'll be happy to correct it. The duplicate form issue, however, is being resolved, so there's no need for you to write in. If there are any further issues, the team will be sure to contact you.
I personally only received one 1099 form, that I am aware of.
Forum discussion at WebmasterWorld.