  We discuss the ghost freeness in the case when we add matter coupled to two
metrics to the ghost-free bigravity. In this paper we show that the
Boulware-Deser ghost generally revives in the presence of doubly coupled matter
and that ghost freeness strongly restricts the model of kinetically doubly
coupled matter. This result may anticipate difficulties in the attempt to
derive the ghost-free bigravity as a low-energy effective theory, starting with
a model applicable at high energies.
