Yeah they will do that - I have a favia about the size of a fist that was doing great for about 6 months and then the clowns started hosting it. Within weeks there were 2 stripes straight through the tissue down to the skeleton where the clowns were rubbing. Basically, the entire center of the coral is skeleton, and the outside of the coral is perfectly fine - basically donut shaped. I was going to sell it until they started hosting, and now I am just keeping it and not moving it because I don't want them to destroy anything else. It is basically the only "cheap" coral left in the tank - and luckily the only one they are hurting.
As long as they stick to one coral, I wouldn't worry about it. They will destroy it, but as long as you keep it there and they like the spot, they won't destroy anything else. If you like the duncans, either frag a piece off it so you have a second colony or kick the clowns out of the tank. You can try removing the duncan from the tank and put it in a friends tank - but they will probably just pick a different coral and may go back to the duncan once it is available again.
Try moving the coral to a different place too - they may just like the location. Switch some zoas to where the duncan used to be and see if they will host those instead - I doubt they can kill zoas.
