2 wires on C comm at furnace
Go for it

The terminal strips on HVAC control boards are designed to clamp two wires under the same screw, 1 on each side.  So, simply make sure the ends of the unused blue wire are stripped of insulation (and strip them if they aren't), tuck one end under the COMM screw on the furnace control board on the side opposite the existing red wire under that screw (you'll need to loosen the screw then tighten it back down to do this), and then you can install your Nest, using the blue wire as the C wire for the Nest.

As to the other wires in Y and C

The other wire in Y and the existing wire in C go off to your outdoor unit -- they carry the signal from the thermostat to the outdoor unit to turn the compressor on.  (In fact, one of the great hints for people with A/C who can't find their C wire is to look for where the air conditioner wires join in -- one of them will connect to the Y wire from the thermostat, and the other will always be connected to C.)