[jmhorn]

The duplicate function in Organize only works one at a time.  A duplicate does sound like what you want.  I have used it when I have item duplicates, but they are not in the same condition.  I use it for other reasons similar to yours.  The process is a pain.  In addition to the one-at-a-time limitation, none of the item fields are set (zero Have quantity, blank collection, etc.).  It also sets the condition to Mint, which is an odd condition to default to.

This may be an unorthodox suggestion (I have not tried it, but I would be willing to):

Traded for cards are added to your Organize and placed into your Recently Traded collection.  A duplicate is created if you already have the card you traded for.  If you could convince another user to add the set of cards you want to be duplicated to their Organize, then construct a transactional-only trade (all the cards you want added to your Organize for a single common card that will be added to theirs), I think it would accomplish what you want with little cleanup once the trade is completed.