Can several Ethernet PHYs cause damage to one another if they transmit on MDIO bus simultaneously?
The MDIO bus is not open-drain, when a device transmits data, the pin will change to push-pull output.
So yes, it can cause damage, but as the outputs will not be constantly driving the pins to opposite logic states, there is less likely to be damage.
Fix the address on one of the PHYs to something different and check board operation.