18. A recommendation system, comprising: a recommendation engine configured to generate personalized item recommendations for target users based on item preference data stored for said target users; and a similar items filter configured to filter the personalized item recommendations generated by the recommendation engine based on item similarities data to prevent at least some items that are fuzzy duplicates of each other from being recommended in combination to a target user, said similar items filter thereby increasing a diversity of personalized recommendations presented to target users; said recommendation engine and similar items filter comprising computer hardware that executes software.
21. The recommendation system of claim 18 , wherein the recommendation system is configured to use behavior-based associations between items to generate the personalized item recommendations, and the similar items filter is configured to use attribute-based similarities between the items to filter the personalized recommendations.