Steam Trading Cards – Do Game Demos Provide Them

You certainly can find the same card twice, or even three times! And it does, sadly, count toward your card drops.

I managed to get 3 Wolfgang cards from Don't Starve, a game that only gives you three card drops.

Also, you can trade for duplicate cards. I traded my Portal 2 Destruction card to a friend who was then able to trade it back to me, despite my having a second Destruction card in my inventory.

Yes,I bought an item for Dota 2 online rather than in-game shop and my progress towards the card increased,don't see why it would be different for TF2.

When you buy items online the system asks you to log-in with your steam account to finish purchase;in-game store is essentially just a simple game web browser to display the original online store(Why?It is easier to maintain one store than 2).So in the end whether you buy something online or in-game it goes thru the same account and the same system.

Also the system is retroactive,meaning all things you bought before trading cards will be counted as you bought them now.So the system will checks your receits and they are the same for both stores.

If you want to check for yourself you don't have to spent 9$,you can purchase the cheapest item you find and then check if you "progress towards next card" has increased.