I am confused about the QM phenomenon of collapse. For example, if an electron is prepared by a magnetic field as spin right and then measured in a device determining whether it is spin up or down, it is said that the electron, which had been in a linear superposition of up and down will now collapse to up or down, with probability amplitudes (PAs) given by the inner product of appropriate bra and ket vectors and probabilities being the square of the PAs. In this case it has a 50% chance of being measured as up. OK, but I am told that post measurement it will always in future be measured as up. Why cannot the measurement that caused the collapse be considered another preparation of the particle in which case it would have a new set of linear superpositions? Or alternatively, what is magic about the preparation of the spin that gave it superposition?