If you have more than one card for a contact, you can merge the cards. Merging cards combines the cards and keeps all the unique information in a single card.
If you have duplicate contact cards in different accounts, you can link them, which retains separate cards in each account, but displays the combined information in one card.
Choose Card > Look for Duplicates.
If duplicates are found, you can merge them into a single card.
Hold down the Command key and click the cards.
Choose Card > Link Selected Cards.
One card appears in Contacts, but the individual cards are still in their respective accounts.
To see the individual cards, click the accounts next to the Cards label.
To change the contact’s name, click Cards, then choose a name.
If you remove an account with linked cards, the cards are simply unlinked, and only the cards in that account are removed.
Command-click the cards you want to merge.
Choose Card > Merge Selected Cards.
Contacts creates a single card.