Search engine spiders can be very forgiving with a lot of duplicate content issues. I’ve found that, given enough time, the engines learn when two websites or web pages are complete duplicates of the other. Once they figure that out then they basically understand that a link to one is a link to the other, etc. One version will ultimately be dropped from the index in favor of the other.
There are two basic problems with this. First, it all takes time. Until the search engines figure out which dupes should be “merged” you’re essentially splitting link flow. Two inbound links, one to each version, produce only have the power than two links both pointing to a single version.
The second problem is that you leave it to the search engines to decide which pages or site should be dropped from the index. When you let the search engines decide, you lose essential control.
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