If you need to include the same category in two (or more) menus for any reason, you can do this as follows:
On your storefront, click on the category you want to copy.
Note the URL of the category page when it opens. This should be something like this: "http://www.yourvolusionstore.com/SearchResults.asp?Cat=23".
In your Admin Area, go to Inventory > Categories and create the new category in the required menu. This Category can have exactly the same title as the original if you want.
Under Advanced Settings > Misc, enter the last part of the URL from step 2 (e.g. "/SearchResults.aspCat=23") into the Alternate URL field.
Note that the category is not really duplicated here; the link is. So when a customer clicks on the link it will be as if they clicked on the original.