Delete duplicate contacts
If you are as disorganized as me then you might often end up with more than one map to the same person (often with different phone numbers and email address in each card). If you have hundreds of contacts to find and manually merging these reproductions can be a pain.
An easier way is to choose Card > Look for Duplicates. This automatically rummages through your contacts, identifying cards with the same name. You can choose to merge any identical cards, and also whether to merge cards with the same name but different contact details (the contact details from one card are added to the second and the first card is deleted)
You can manually merge cards too. Shift/Command-click the cards to merge, then choose Card > Merge Selected Cards.