Giving away partially owned bundles

Most Steam packages do not give extra copies if you own some of the titles already (notable exceptions are Valve's packs and a couple others like Flatout).

When I add XCOM EU Complete to my cart, it gives the warning:

If purchasing for yourself, you will not receive an extra copy of:
XCOM: Enemy Unknown

If you buy it on Steam, the base game will be wasted when you activate the pack, you don't get an extra copy.
You could try looking to buy it somewhere else, maybe some stores sell it with separate keys.

afaik no. only valve complete pack gives you extra copies. once you redeem xcom complete pack you won't get the extra copy of base game. buy enemy within for yourself other dlcs are just cosmetics.
this is really problem with bethesda goty vs base games

I can tell you from my own experience that you can't split up that "XCOM complete" package. Just like you, I already owned Enemy Unknown. When I figured out that the expansion (Enemy Within) costs basically more or pretty much the same as the complete package which comes with two extra DLCs, I bought that one. I even bought it as a gift, hoping that I would somehow be able to split it up, but alas, as the store states, you don't get any extra copies, you only get one copy of the whole pack, that simply ads the stuff you don't already own to your library.
I think XCOM is an example of a very bad pricing policy, where the expansion DLC on it's own is more expensive than rebuying the whole game, thus punishing early buyers who got the game since before there even was an expansion. The only good thing's that it's not that expensive anymore anyway.

The XCOM: Enemy Unknown was given out for free anyway, so it doesn't have much value. Though I just checked and apparently it's still valid to give on SG for some reason, but it'll at most count as bundle. So basically, consider buying the "Complete Pack" the same as just buying the DLC, in which case you might want to consider just buying the DLC separately, since the other would be a waste of money.

Thanks for all the answers, you are really helpful :)