A lot has been made of penalties for duplicate content, but as long as it is judged to be duplicate content for legitimate reasons, there should be no penalty. You ask, what constitutes a "legitimate" reason? For example, if your goal is to expand from a US market and you want to target a new market in the UK, then a sister site with duplicate content and alternate spellings (color, colour, aluminum, aluminium) may be a necessary adjustment. It may be best to set these sites up with a region domain, expanding from "stuffyoucanbuy.com" to "stuffyoucanbuy.co.uk." This would be considered a legitimate excuse for duplicate content.

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