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Many websites have incorporated the necessary development that allows query parameters from their internal website search to be indexed by search engines - with the view of essentially driving more traffic - however untargeted it is likely to be - it also builds many thousands more indexed pages from their domain in the search engines. A common way of doing this is by monitoring incoming search term referral traffic and then adding a widget on page to contain these search queries linking to the same search query processed via their internal search, hence how so many of these pages can be indexed for a particular domain. It's very common with torrent / file sharing websites. You unfortunately can't control what methods another website is using but it sounds like there is more to it in the sense of what they have done if they are outranking for a completely unrelated term to their domain yet have identified that your domain is authoritative for this same terms. Without knowing the specifics, it's hard to elaborate further on this... Happy to look if you wanted PM domain/keyword.