Perhaps it may be easier to think or remember that you do not want two sources to ground for any one system. Think of your standard dist system, there is only one ground (at the plate/rod or water main) everything else ties back to the panel…therefore they are bonded to ground.
Just use this basic explanation and apply that to more complicated systems. When you start talking about “isolated grounds” which really should be called “isolated bonds”, you are isolating them from other metal parts that are also bonded and you are connecting it directly to the grounding system. In this case, you are not duplicating the ground but are bringing a bonding wire directly back to the ground system.
Hope this helps.
Cheers
