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Duplicate content does not have to involve pages that are grouped together. You can even have cross-domain duplicate content. There is not a direct penalty for duplicate content. The issue is that when Google sees pages (wherever they may be) that appear to be the same, it will choose which one to index, omitting the others. So as Andy says, when pages are being evaluated for duplicate content, it is best to be aware of very similar pages so you can take steps to distinguish them, if you want them each to rank.