It's inevitable that you will occasionally find yourself with duplicate entries for the same contact. Fortunately merging the information into a single entry is relatively simple.
In either the Phone or Contacts app, open the duplicate contact and tap Edit in the upper right corner.
Scroll down and select Link Contacts.
Open the duplicate entry and tap Link in the upper right corner.
If you have more duplicate entries, select Link Contacts and repeat.
Tap Done once you have linked all duplicate contacts.
To unlink contacts, simply tap the red minus sign next to the contact you wish to unlink.