  We prove a U-spin amplitude triangle relation among doubly Cabibbo-suppressed
(DCS) charmed meson decays, $D^0\to K^+\pi^-, D^0 \to K^0\pi^0$ and $D^+_s \to
K^0 K^+$, congruent to an isospin relation among corresponding Cabibbo-favored
(CF) decays. U-spin breaking in relative phases between CF and DCS amplitudes
affects time-dependent studies of $D^0-\od$ mixing. Comparison of final state
phase patterns in DCS and CF amplitude triangles, which can shed some light on
these phases, is carried out in a phenomenological framework incorporating
resonance contributions.
