You are able to merge customer accounts together under the "Customers" page.

Search on the accounts that you would like to merge, from there, if the accounts are ready, select on "Actions" > "Merge Customers":

That should allow you select which accounts you would like to merge:

After selecting the accounts (it should be highlighted to show which accounts were selected), click on "merge 2 customers" button:

This should now allow you to edit or adjust the customer's information, such as the name, email address, phone number, etc. Select on "Complete Merge" to save the changes made:

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