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Template:Duplicate category/doc
From OeisWiki
Duplicate categories are categories relating to the same concept under different names. It is only when the categories that we consider duplicate contain protected pages that we need to categorize them so (so that someone with the the proper edit rights can merge them), otherwise we can just merge the categories ourselves.
To invisibly categorize a category as Category:Duplicate categories (a hidden category) you just use the template {{Duplicate category}} in the category page.