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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.
Same sort of thing if both .com and .co.uk are showing the same content. The "Strength" of all pages is reduced.
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]