Not sure what "differences" you are wanting to incorporate between product. Seeing that you've been a forum visitor for as long as you have, I hope you don't take offense to a few things I'm about to suggest. It is in the goal of verifying familiarity with the options available and perhaps honing in on what it is that is needed but not available out-of-the-box.
If the exact same product is desired to be shown in multiple categories, there is the availability of creating linked product. (From admin, copy the product as linked to the desired categories or use the multiple category tool available at the bottom of the right hand side when initiating the copy.)
If the difference(s) are related to characteristics of the product, then there are attributes that can be used. The attributes offer ways to slightly modify the product including its cost, shipping weight and as applicable image information.
To create unique products, the product can be copied and then modified to suit.
Now outside of the base software, there is the rugged SQL query method that could be used with extreme caution to duplicate product somewhat en masse.
Then there is software available that can be used that will support editing the associated data in a spreadsheet.
So, based on these items, is there something that seems to fit your need or is there still something else? (mind you, any of those features could be "shown" or activated using a single button referred to as sell similar.)
