"The occurrence of conjoined twins is rare. Its actual prevalence is unknown, but it is estimated to range from 1:50,000 to 1:200,000" [1]. 1 in 200,000 would raise it to 99.9995%. But as pointed again and again in the other comments, the pointless, hyperbolic figure is irrelevant. When cutting the planarian worm head, the regeneration is always, 100% a head, if no change in the bioelectrical gradients. The argument was about the deterministic computation done by biology in the morphospace.
[1] Importance of Angiographic Study in Preoperative Planning of Conjoined Twins Case Report, https://www.sciencedirect.com/science/article/pii/S180759322...