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I'm seeing that our site rose strongly over 5 days and then has started to fall away, as though after the pages came up in the search, G is going through them and filtering them out. The pages that are most "similar" are at the 4th and 5th level, which started to kick in 5 days ago before the drop of 2 days ago.
This is part of the problem discussed over here: Duplicate Content on Localised Search [webmasterworld.com]about multiple TLD's Duplicate Content on Localised Search, but likely applies to other forms of content related duplicate content.
I want to put redirects in place to counteract this, but wondering if G axes a whole site or just the duplicate pages. Looks like a whole site to me, but I'd be interested to hear from other's experiences.
I believe that Pagerank distribution is affected within a site; and I see that could have a knock-on effect in several ways... and again, talk about PR stuff links in with some of what Matt Cutts has mentioned in relation to Supplemental Results in more recent times.
[edited by: g1smd at 6:42 pm (utc) on May 25, 2007]