If you have two customers with the same details possibly duplicated and to which you have invoices applied to each, you can merge the customers by updating the customer name on the invoices. Therefore all the invoices will end up under the original customer details and you can delete the duplicate customer details.
Merging the Customer
To do this go to Customer > Click the duplicate customer > Invoices, Quotes & Stats > Click on an invoice > select the Customer drop down list in Invoices details -> Select the original customer details -> Click update details. You have now successfully switched that invoice from the duplicate to the original invoice. Repeat this as many times as required and then delete the duplicate customer details. The exact same process can then be followed for your suppliers if you have an issue with your suppliers.