Wiring a New Outlet from an Existing Outlet

I'm just looking for verification here. My existing outlet above my work bench has two black and two white wires attached, with two 12/2 wires entering the box. I'm going to add an extra outlet to my deck (GFCI, of course as is the circuit) which will involve running wires downward about 3 feet and into an outdoor rated box on the deck. Is this as simple as connecting the three blacks (including the new 3' wire) and then the three whites and running a pigtail black and white to the existing outlet? I just want to make sure I don't lose anything else on the circuit. Thanks in advance for your assistance.