1. All Duplicates: System displays a list of contacts with a medium of high chances of a match. To trigger the option please click on the name of the contact.
2. Merge Contacts: This is the function I recommend you to use if you have the ID of the contacts you wish to merge. The ID you enter in the first Search By ID - To Contact located at the top is the contact you wish to keep and the ID Search By ID - From Contact is the contact that will be merged accordingly. Make sure the 'Search' button is used in both functions to trigger the search.
This function also allows you to delete the contact on the second field by clicking on the Delete after update option as indicated below.