When needing to add multiple of the same material to an invoice, my technicians have found that it is easier and faster to not click into the material to change the quantity. Instead, on the material list, they just click the 'Add' button on the right. This adds a new line item of Qty 1 to the invoice, and I'll get multiple line items of the same material. I can (and have) told them 'til I'm blue in the face to click the item and put in the quantity, but it never happens.
Idea: when clicking the 'Add' button, if the material is not currently on the invoice, it will create a new line item of Qty 1. If the material is on the invoice, it will increase the qty of the existing line item by 1.