This is a setting in Tools > Options > Other > Printers. If Printer #2 has 1 Copy highlighted, this will print to both printers. Change the setting here to 0, and only one invoice will print.
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, if Printer 2 is set to print 1 copy, when emailing a hard copy will always be printed. Set this printer to 0 if a hard copy is not required.
This usually indicates a problem with the invoice report, invoice script, printer configuration, or invoice data being processed during printing or previewing.
There is a security setting under Tools > Options > Job > Invoice to allow users to invoice customers on automatic credit hold.
Note: Regardless of whether the user has security to invoice customers on automatic credit hold, they will not be able to do so on manual credit hold as this is far stricter. The only way to overcome this is to place the customer on auto credit hold.