Dr Dukan wouldn't advise using protein replacement shakes, the point of the diet is to re-educate us into a healthy diet, the problem with protein shakes is they can become a crutch so you effectively turn Dukan into slimfast....then you miss out on the transition into conso/stab and could undermine your efforts to keep the weight off long term.
That said I've noticed some people do supplement with protein shakes - wouldn't be advisable though really xx