When it comes to search engine results pages (SERP), diversity is important. Diversity gives the user the opportunity to find the information or services that are more tailored to their specific needs. Having one domain dominate the SERP might not give the user the best quality result they are looking for.
Changed on June 6th, Google has stated that moving forward, only two listings for the same domain and sub-domain will be shown per page for most search queries.
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