Electric Hot Water Tank Wiring

My mother lost her free gas, so I bought an electric hot water heater for her. I ran the 10-2 from the box, went to wire it up, and when I opened the metal plate covering the wires on top of the heater, I found 2 green wires instead of one. One is screwed into the metal dish where the wires come out of the heater, the other goes down through the grommet into the heater, along with the red and black wires. I haven't seen this before...just want to get the wires hooked up correctly. The manual for the heater only shows one green ground wire...

Does one get attached by screw to the metal cover that goes over the wires, and the other attached to the ground from the box?

I have a pic, but don't know how to attach it here, if that's even possible.

thanks!

Electric Hot Water Tank Wiring