duplicate contacts problem

duplicate contacts problem

Before I import my customer data from zoho crm, there is already a customer I created manually inside zoho books.
Therefore, there is a duplicate contacts. 

I wanted to use the contact imported from zoho crm and delete the one I created manually, but there are some closed invoices for this contact.

How can I merge the 2 contacts or to import all the invoices from the manually created contact to the crm contact?

thanks