There are a couple of things to be aware of in relation to printing invoices.
Why are Two Invoices Printing?
This is a setting in Tools > Options > Other > Printers. If Printer #2 has 1 copy selected, this will print to both printers. Change the setting here to 0, and only one invoice will print.
An Invoice is not Sending via Email
This is a setting in Tools > Options > Job > Invoice. As per below, ensure that Print Invoice after invoicing a job is ticked. If not ticked, no invoice will print or be sent via email.
Further information: