The Doppelfleish is a strange surgical biomancy tainted by Slaanesh.
It causes one half of the body to turn into an exact replica of the other, but made out of living metal. But the taint causes the organic side to rot over time and emit the stench of corruption. Those who's bodies reject the surgery are left with a broken anatomy.
During the Age of Myth this surgery was the perfect solution for the Demian Seers obsession with symmetry. It was highly fashionable, but as the taint causes their organic bodies to rot, forcing them hid it with pomanders and perfumes. Those whose bodies reject the surgery, went into a panic to find a solution to their now broken anatomy. The Herald of Slaanesh Litheclaw used this to corrupt the seers with promises of salvation through Slaanesh.