From time to time, contacts may appear twice in the Contacts app on the OnePlus smartphone, which is caused by saving Messenger apps, such as WhatsApp.

To keep track of the contacts list, we can merge duplicate contacts by selecting both.

Android 10 | OxygenOS 10.0
Step 1:  Open the  Contacts app
Step 2:  Tap and hold a  contact
Step 3:  Select the duplicated  contact
Step 4:  Open the  menu
Step 5:  Tap on  Merge
  1. Open the Contacts app
  2. Tap and hold a contact
  3. Select the duplicated contact
  4. Open the menu
  5. Tap on Merge

OnePlus Instructions