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I have a project with 8 domains and each domain is showing the same content (including site structure) and still all sites do rank. When I search for a specific word-string in google it lists me all 8 domains. Do you have an explanation, why Google doesn't filter those URLs to just one URL instead of 8 with the same content?

In a bigger project are several domains that show the same content like the main-site (there is a reason to have it like that). Now those "double-content domains" are indexed and ranking in Google. But now I see that all those double-content domains have no pagerank visible, despite they do all have their unique own backlinks. Do you know why those domains don't show Pagerank? Can it really have something to do with the double-content situation?