---- Very Odd - Big Drops in SERPS Today April 26, 06
jadebox - 2:08 pm on May 9, 2006 (gmt 0)
This however can't cause a penalty on an entire website and it doesn't - simply the second (third.. etc) url is put as supplemental result.
It won't cause a penalty, but may reduce the ranking of a page. If Google sees www.domain.com/page1 and domain.com/page1 as two seperate pages, Page Rank may be split between them since some incoming links will use the "www" and others won't.