It's not a good idea to have multiple copies of the same product just to show that product in multiple categories. This will create a mess in the store, especially if you manage your inventory levels and your pages will be downgraded by Google for the duplicate content.
Much cleaner and safer way is to assing a product to the multiple categories using "Additional categories" section:
see screen shot: http://screencast.com/t/d86wfWuapD
or if you are importing your products, add extra categories under the main one in the import file:
see screen shot: http://screencast.com/t/GPFGRQ2q