Just a question about Steam keys/gifts

not sure, but I'd expect the worst and you'd lose the extra base game

If it is specified on the game page that would give you a gift copy it would, else just the dlc will be added. I think Valve games add a copy to the inventory.

pretty sure you just gain the DLC's, if you get (giftable) copys it's stated on the store site afaik

Clicky

As far as I remember, you only gain the DLCs and the base game part will be lost. There are only a few games that give you another copy when buying them in a bundle e.g. Half-Life 2 and epizodes.

The only games I have witnessed giving extra copies as described are Valve and Daedalic titles.

In all but a select few cases, no

Unfortunately no, you will not receive an extra copy of Skyrim if you own the base game and activate the Legendary Edition.