There are different ways of obtaining cards. Whenever you win a challenge, a Courier brings you rewards that include cards, among others. Another way of getting cards is by purchasing them in the game Shop using your Coins.
You can get repeated cards, but you can use them to upgrade the unit the card represents, as you need to collect several of each in order to be able to improve them.
Some cards will be harder to find than others. There are four different rarities: Common, Rare, Epic and Legendary. They can be differentiated by their color.