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Great answers here. Keep in mind the idea of "keyword stuffing" is a fluid concept. A well-written Wikipedia article may use the same keyword 60 times without being flagged for stuffing. Context is key. If it makes sense to use your keywords multiple times than it's usually fine. The biggest violators that we usually see are in the title tag "Buy Cheap Florida Homes - Florida Home Finder - Your Best Florida Home Broker" - ouch! As for plurals, Google will rank different pages differently based on what they perceive as searcher intent. I don't know of any hard and fast rules here - some plurals are considered synonymous with the singular version, while other times it can make a difference in ranking.