Motor Mounts Broken?

Thanks
My authorised MB Motor shop in Aug replace only 1 mount because when reversing (gear on Reverse), the car vibrates badly. But he check when on Drive, it do not vibrate.
So only 1 mount was replaced in Aug'05.
Yesterday the other motor shop who replace the water tank & Radiator, spotted both mounts broken.
The question is either he presume 2 need to replace at same time without realising the other shop had replaced last yr or previous Shop had done something to stop the vibrating?
Currently on Drive i dont notice much vibration even on stationary. I suspect its the right mount which was replaced, am i right?
Some 1 mention if not replace it may cause stress upon other parts of engine, what are those possibilities??

Kind Regards

Thanks
Correction, the L/H mount is the one seen rise& lower during the Reverse & drive gear (with brake on). Whereas the R/H mount hardly any movement, meaning its collapse, correct?
You mention oil contamination, can damage the mount? How is that?
Also I notice in my radiator/water pump leak previously the colour of the coolant by the previous authorised MB service shop is green colour. Some post mention that MB engine dont like green coolant. Now with the new Raidator/water pump in place, I notice coolant in reservouir is not green, look like lemon colour I guess.
Kind Regards