Emailing Sales Invoices - duplicate email address

Recently I have noticed that when I email an invoice to a client it puts their email in the cc box as well as the TO box.  I have to delete the cc each time.  I cannot untick the cc box under the customer details as it is greyed out.  It makes no difference whether there is another contact set up, it will still put the main contact address in the cc box too.  I contacted Sage via online chat and they suggested that I set up another contact with no email.  That was useless as clearly has no bearing.  I did point out that I didn't want to do that for my 1000 customers!  Anybody else having this issue.  It's driving me mad!