As a fundamental component for a quantum computation, the measurement needs to be implemented in a fault tolerant way.
As indicated in Chuang and Nielsen Quantum Computation and Quantum Information, a quantum circuit that allows the implementation of a FT measurement is the one represented in Fig.1:
However, three measurements are needed in order to make this implementation actually fault tolerant, due to the possible presence of errors in various part of the circuit (as it is said in the book, between two CNOT gates in a verification step for the cat state).
But how is it actually possible to execute three subsequent set of measurements? Isn't the data (the states on which the measured is performed) modified by the application of the measurement operation? If that is the case, are the measurement operations applied on different data?