Duplicate keywords happen when one of two or more of your ads are using the same keyword list. It's best to avoid having duplicate keywords in your account. Google shows only one ad per advertiser for a particular keyword, so there's no need to include the same keywords in different ad groups or campaigns. It’s okay to have the same keyword in different match types in the same account if you want to use different bids or creatives.
Before you begin
For more background on adding and refining your keyword list, review how keywords work.
Duplicate keywords with different targeting
For more basic accounts, it's best to use a keyword only once, including variations of your broad match keywords (as well as your phrase and exact match keywords if you haven't chosen to narrow your phrase and exact match targeting). For example, the broad match keywords “red car” and “car red” are duplicates and the keyword with the higher Ad Rank will be used. With overlapping or duplicate keywords, you’re not bidding or competing against yourself.
For more advanced accounts, it's a good idea to keep your keywords unique within the same campaign. You may want to duplicate keywords if you have separate campaigns for different targeting or individual geographic locations. These won't directly compete with each other, so someone searching for "chocolate" in New York can see your New York store location ad and someone searching for the same keyword in New Jersey can see the one for New Jersey.