Duplicate describes something that's an exact copy. If you tend to accidentally lock yourself out of your car, it's a good idea to carry a duplicate key with you just in case.
The word duplicate is useful when you're talking about a copy of something, like a duplicate of your birth certificate that you keep in a file, or a duplicate driver's license that you apply for after losing the original one. The verb duplicate is pronounced differently, with a long a sound, and it means to make a copy of or to multiply times two. The Latin root, duplicatus, means "to double."