Open "Contacts" app :
- tap on ,
- tap on "Suggestions", if needed,
- tap on "Clean up duplicates" or "Merge & fix".*
* This option is displayed only in the presence of duplicate contacts.
To merge contacts separately, tap on "Merge" into each duplicate's window or "Merge all" with a tap at the bottom of the screen.
NB: Merging contacts of different accounts is not possible.