What i can do with double cards?

Sell them because money.

trade them for the ones you need, or sell them. Thats pretty much all you can do, also you can make gems with them but i wouldn't recommend it because sell = money

Go to the steam forum for the game. There's almost always a trading subforum with people searching for cards.

Try this if all else fails

http://www.steamcardexchange.net/

sell the duplicates and then use that money to buy cards you want.

I say sell them because most people will only accept 2 of your cards for one of theirs.

Unless i find a friend who also has doubles of the set, i sell them. The two cents i save isn't worth the hassle of going through trade.

I use Steam Trade Matcher to trade duplicates.

There are also several dedicated "trade bots" on Steam, as well as regular users that run a trade bot on their account when they aren't using it to play games.

SteamCardExchange.net is pretty neat, and also runs a trade bot (special rules apply, though).

You can also try this: http://steamcommunity.com/id/swappy
It's a trading bot and do 1:1 card exchange for the same game, so a good way to get the card you need for a duplicate card, however it's game list not complete so you can't find cards for many games, but it worth to try.

I myself every time check it first if I have this kind of problem.

sell on market or trade for another cards or save it and craft badges....

http://www.steamtradematcher.com/

join here people send you offer for your duplicate cards if you dont want to waste time for it :)

F5 because...

I wanted to mention, i looking for cards for various games, mostly i want for Super Sanctum TD, Saints Row IV or Borderlands Pre-Sequel. At the moment no luck