If you have two entries for the same customer, first, edit one of those entries to add “(duplicate)”, or something to that effect, to the end of the name.
Then, scroll to the bottom of the duplicate customer’s page to see all of the invoices that are assigned to that customer.
Click on each one, change the customer name on each to the correct name of the non duplicate.
Once you changed the name of the duplicate customer to the new original customer “click Save”
Once there are no more invoices assigned to the duplicate customer, you can return to the customer list and delete the duplicate by clicking “X”.
If you click on the new original customer and scroll down to the invoices, you will see that the invoices of the duplicate that you deleted are all there.
However, we recommend that, instead of deleting, you just change the name of that customer to “_DO NOT USE”, or something like that and make a new customer with the original name.