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This is a common problem in WordPress. The tag archives, category pages and author archives can create many multiples of the same content and cause numerous duplicate content issues. Most people choose to NoIndex their archives since they have potentially little Search value but they do/can have value from a User perspective. Setting posts to show snippets on Archive pages can lessen some duplication issues as well. You should also double-check the tags you are using as one-off tags can cause a lot of problems with duplication. If a post has 7 tags on it and those 7 tags are only being used for that one post, you now have about 10 versions of that post sitting on your site. Make sure to only use relevant tags and that the tags used are then also used as needed on future posts in order to tie together related posts. All the functionality to NoIndex archives and change posts to snippets can be found either as part of your current theme or the Yoast plugin is a good download to help manage those things.