Merging Customer accounts is a function that may be needed if two customer accounts were created for the same customer.  Instead of deleting a customer and all the invoices, contact, comments, in-store credits and orders, simply browse to the customer account you wish to merge into another customer record, and double click to open the record.

On the top left area of the customer record, click on the Merge with Another Customer button

This will open the following window:

Note: Read carefully the text in the blue box to understand what you are about to do, as this action is not reversible. 

From the drop down list, select the customer (See Customer Accounts) you wish to merge with. The customer listed in the drop down list will be the primary customer that will remain active. After you have selected a customer below, click on the merge button to process the merge request.  If you do not wish to proceed with the merge, simply click the cancel button.