Before initiating merge, Identify which one is master and which one is duplicate
-  Identify and note down the Customer IDs of all duplicates (including the master customer) 
-  From the list identify the Customer ID which will act as a master ID. Usually select the ID that has maximum correct information, or maximum sales against them, or the one that is oldest. This is decision of the person who will merge. 
Execute the plan:
Steps to change the parent ID:
-  Open the customer record that is identified as duplicate 
-  In General Tab: Click the Edit button, a Popup will open where information can be edited. 
-  Under column Parent ID: Paste the Customer ID of the Master Customer 
-  Click confirm to close the popup 
-  Click “Save Document” to save the changes to the customer record 
-  Repeat steps for each duplicate 
If done successfully, the duplicate customer will show the parent id name and the record will move to the “Duplicates” Tab in the search screen