Isn't this built-in? - Try this:

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The software actually supports this feature - it's built in. Look in your admin, where you add/update/edit products and categories, you may choose a product and then one of the buttons on the top right will read "copy" - press it and it will give you a chance to copy the product to another category (any, it could be parent, child, top or unrelated category). It will give you a choice between linking to the same product on multiple categories, or actually create duplicate copies that can later on be edited seperately. Is this what you are looking for?