Open the Contacts app or the Phone app and tap Contacts
Find the duplicate contact you want to merge
Open one of the duplicate contacts
Tap Edit
Scroll down and tap Link Contacts
Select the matching contact you want to merge
Tap Link in the top-right corner
Tap Done
Repeat for any other duplicate contacts
To merge contacts from iCloud, turn on Contacts in iCloud settings and let them sync
To remove duplicates automatically, open Contacts and check for the Merge Duplicates prompt if available