All cards have their own individual QR code that isn't shared by any other card. The exception is standard art token cards.
If a card is unowned (if it was just opened in a booster pack for example), it can be registered on the official Altered app and added to your collection. We recommend scanning new cards and securely registering them to your collection as soon as possible.
If a card is scanned and already registered in an account, you won't be able to register it to yours. Instead, it gives you access to the card's information: text, rules, lore, marketplace value, etc.