You can send Card duplicates to your in-game friends. And likewise, your friends will be able to do the same to you. Here's how to send a card to a friend:
➡️ In the main event screen, tap on the set with the card you wish to send
➡️ In the set tab, navigate to the card you want to send, and tap on it and press Send
➡️ In the pop-up window that opens, choose the friend you want to share with, and tap the Send button next to their name

Please note:
- It's not possible to send Gold cards, even if there are duplicates
- If neither of your sets has duplicate Silver cards that you could share, the Send button will show as incactive
- You can send Duplicates from both completed and incomete 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.