By default, Magento does not provide this type of feature in the backend. With the help of this Merge Customer Accounts Extension for Magento 2, you can merge any customer accounts together. For example, you could use this extension to merge two customer accounts together into one. Sometimes this happens when old orders using an email address no longer in use by the customer exist and then the customer has purchased again recently with a new email address and wants to combine order history and other information together into the new account.