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Be very careful about making assumptions regarding competitors. Just because you see one thing, does not mean either that one thing is helping or hurting a site. SEO is a vast, complex environment. If a site has enough very strong signals across many areas, one or even a few very poorly executed things may not hurt the site. Or it may not hurt the site "until Google catches up with it". Duplicate content, regardless of method (within a single site, across multiple domains, across a mix of domains and subdomains" is never a true best practice.  Ever.  it's artificial, and Google most definitely takes the position that if you are attempting to "artificially" (in their view) manipulate rankings in a non-best-practices manner, that's a violation of their guidelines, policies or TOS.