There is NO Copy / Paste function in the Scen Editor.
In the new ANW Scen Edit, there is a new function. After you insert a unit (plane, ship, sub, facility, or installation), you can hit Cntl+A and then click again on the map. This will insert a second unit exactly like the last one you just added.
Unfortunately, it will only insert a unit from the Database. This means that if you insert an ammo dump, load it with weapons, and then hit Cntl+A, the next ammo dump to be added will be unloaded.
The PlayersDB will be trying to make this even easier in the future with pre-constructed Installations. This way, you can insert a completely pre-fabricated base at the click of the mouse.