  We study the procedure for sequential unambiguous state discrimination. A
qubit is prepared in one of two possible states, and measured by two observers
Bob and Charlie sequentially. A necessary condition for the state to be
unambiguously discriminated by Charlie is the absence of entanglement between
the principal qubit, prepared by Alice, and Bob's auxiliary system. In general,
the procedure for both Bob and Charlie to recognize between two nonorthogonal
states conclusively relies on the availability of quantum discord which is
precisely the quantum dissonance when the entanglement is absent. In Bob's
measurement, the left discord is positively correlated with the information
extracted by Bob, and the right discord enhances the information left to
Charlie. When their product achieves its maximum the probability for both Bob
and Charlie to identify the state achieves its optimal value.
