You can send Duplicates of your Silver cards to your in-game friends and teammates.

Here's how to send cards:
➡️ In the main Collections screen, tap on the set with the card you wish to send.
➡️ In the set tab, navigate to the card with duplicates that you want to send. Please remember that you can only share cards that have duplicates: such cards will have a badge in their left corner indicating the number of duplicates (for example: +1).
➡️ Tap on the desired card and press Send.
➡️ In the pop-up window that opens, navigate between the tabs Friends or Team to share the duplicate with your friend or a teammate.
➡️ Choose the player from the list, and tap the Send button next to their name.

Please note:

• It's not possible to send duplicates of Gold cards.
• If neither of your sets has duplicate Silver cards that you could share, the Send button will show as inactive.
• You can send Duplicates from both completed and incomplete sets. You can still share Duplicates even after you've completed the whole Collection.
• If you don't have in-game friends yet, refer to this article about connecting with your Facebook friends in the game.
• If you aren't part of a team yet, check this article on how to join a team.