It's possible that they were synced to your iPhone by different accounts.
Just an example:
Let's assume you added one of your contacts in an email account. Then you followed the same person on Facebook. The information will show up twice on your iPhone.
In that case, it's better to link these contacts.
If they are from the same account, make sure the information is exactly the same. I had duplicate contacts, where one had the birthday date, and the other one the the telephone number. Linking them would be better than removing them. You'll see the contact once in your Contacts app, and every information will be visible there.
If they show exactly the same information, and there are many of these duplicate contacts, you could use an app to remove duplicates. There are a few available in the App Store.