Trying to install a WiFi thermostat. I have the unused blue wire which I believe is the common wire. It is unused both and the furnace level as well as on my current thermostat. However there is already a white wire in the COM: connection at the furnace. So I have no open terminal.




There's nothing wrong with adding another wire to the COM terminal at your furnace.
Just use the spare blue wire there along with the existing white wire from the other cable.

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    do leave a small note behind the thermostat so as not to confuse the next person that replaces the thermostat – dandavis Jun 20 '19 at 19:06

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