How to attach light with two ground wires to fixture with one ground wire?
Connect the white wire from the ceiling to the white wire on the lamp. Secure it with a wire nut sized for two conductors.
Connect the black wire from the ceiling to the black wire on the lamp. Secure it with a wire nut sized for two conductors.
Connect all the other wires to the green screw on the mounting plate. Use a pigtail construction with a wire nut sized for four conductors.
To make a pigtail, cut an extra piece of wire about six inches long and connect one end of it to the green screw. Wrap the end clockwise around the screw - in the direction that the screw tightens - so that when you clamp down on it with the screw it will be pulled in instead of being squished out.
This hookup wire can be insulated or bare (if insulated it should be green or green/yellow striped). The bare wire on the lamp is ridiculously long so you can cut it in half and use the excess as the hookup wire.
You now have four ground wires: one bare wire from the ceiling, one bare wire from the green screw, one bare wire from the lamp, and one green wire from the lamp. Twist the four ends together and secure with the wire nut.
Each wire nut has different kinds and sizes of wires in it so check carefully. Try to pull each wire out of its nut.
The reason for the pigtail construction is that wrapping three wires around the green screw would result in an unreliable connection.