How do I check to see if an invoice was sent, and re-send an invoice from Duet?

You can go to Billing -—> Invoicing —→ Invoice History. There you can see the invoices that were created, and the date they were emailed to the client. If you’d like to resend it, click on the box next to the invoice to select it, and then use the drop down menu at the top left of the screen and choose “resend/email invoice.”