About duplicate keywords
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, unless you're promoting your mobile app on Google Play. If two of your active ads are using the same keywords, they are essentially competing against each other instead of other advertisers.
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 will compete against each other. Since the better performing keyword will trigger your ad more often, you'll want to remove the duplicate that performs worse.
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 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.