I am installing an ecobee smart
Thermostat

One zone system

The existing old thermostat had 4 wires coming out
RGYW attached and
One spare blue that I could use for C connection

The hvac system panel had
The 4 wires from the thermostat
RGYW and the spare blue
But….
also a fifth connected wire
On the C was already wrapped another white or gray wire….there was not another end of this coming out at the thermostat

Following the instructions I should use the spare blue at the thermostat and the hvac panel to connect to the C

But what do I do with the spare white/gray wire that is already attached to the C on the hvac unit ?

Just to start simple and see…
I tried to just connect the new ecobee using just the 4 RGYW and not change the hvac panel or add any C connection
It did not power on

My next thought is to remove the wire that was on the C on the hvac unit
Connect the blue on both the hvac and the thermostat
That would match the instructions and I think would work

But what do I do with the spare wire that was already on the C ?