Interesting, Never seen a thermostat that has two of each wire connected to the terminals. It looks like they ran two cables in parallel from the furnace to the thermostat which makes no sense to me.
Do you have just one thermostat?
Do the thermostat cable only have 3 wires each?
I wanted to ensure that everything was covered here.
Thanks for the input, @Patrick_Caezza.
I'm sorry for the late reply, wanhanii. I'd suggest contacting an HVAC technician for your Nest Thermostat installation. Incorrect wiring could cause an HVAC issue. With this, we can assure the use of the double wires connected to your existing thermostat.
If you have more questions, let us know so we can assist you right away.