Duplicate Customers

[Agradvice]

Please can anyone help me.  I've just started using express invoice but I've notice the previous user had created duplicate Customer file with different invoice attached.  Is there anyway I can put all the invoices for the customer into just file and delete the duplicate without deleting the invoices as some for the invoices have been paid.  Thank you in advance.  I would really appreciate help on this.

Also how do you write off a customers o/s balance with the customer is deceased