With a large number of contacts, it is quite possible that duplicate contacts are created on the Samsung device by mistake. Even with multiple imports or different locations, contacts can be displayed twice. With One UI, Samsung offers the "Merge Contacts" function. Here, contacts with the same name, phone number or e-mail address are listed. Using these criteria, the contacts can be merged.

Android 10 (One UI 2.0)
Step 1:  Open the  Contacts -App
Step 2:  Open the  menu
Step 3:  Tap on  Manage contacts
Step 4:  Tap on  Merge contacts
Step 5:  Choose between  Number, Email  and  Name
Step 6:  Select desired duplicate  contact
Step 7:  Tap on  Merge
  1. Open the Contacts-App
  2. Open the menu
  3. Tap on Manage contacts
  4. Tap on Merge contacts
  5. Choose between Number, Email and Name
  6. Select desired duplicate contact
  7. Tap on Merge

Samsung Instructions