Do you have another tank in which you can place the duster while you try to catch the peppermint shrimp?
If so, then put it in the other tank, catch the shrimp, and return them to the LFS or give them to a friend or something.
It is possible that they are attacking it because it is sick or dying, but I for one am not big fans of the peppermints, and am trying to catch mine and give them away. There are other shrimps that will not attack dusters, I think. These are not the ones.
It is also possible that they just want to play with the duster, but somehow, I doubt that.
Maybe they're curious.
Why take the chance? My peppermints killed three big, expensive dusters that I bought. They have eaten many of the tiny ones that pop up on my rocks. I am not happy with them at all.
my .02