  Effects of final-state interactions in nonleptonic decays of charmed mesons
are studied in the framework of quark-diagram approach. For the case of u-d-s
flavour symmetry we discuss how the inelastic coupled-channel rescattering
effects (and, in particular, resonance formation in the final state) modify the
input quark-diagram weak amplitudes. It is shown that such inelastic effects
lead to the appearance of nonzero relative phases between various quark
diagrams, thus invalidating some of the conclusions drawn in the past within
the diagrammatical approach. The case of SU(3) symmetry-breaking in Cabibbo
once-forbidden $D^0$ decays is also studied in some detail.
