Duplicate sites or multiple domain names that redirect to the same content without a 301 Permanent Redirect are bad news.
If you must redirect more than one domain to content, the only proper way to do it is to pick one domain--then point all the others to it using a 301 Permanent Redirect.
If you feel you must have more than one domain name, then it's best to redirect using a 301 or have unique content on it.
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