  We find a new $CP$ violating effect in charmed hadron decays into neutral
kaons, which is induced by the interference between the Cabibbo-favored and
doubly Cabibbo-suppressed amplitudes with the $K^{0}-\overline K^{0}$ mixing.
It is estimated to be of order of $\mathcal{O}(10^{-3})$, much larger than the
direct $CP$ asymmetry, but missed in the literature. To reveal this new $CP$
violation effect, we propose a new observable, the difference of the $CP$
asymmetries in the $D^{+}\to \pi^{+}K_S^0$ and $D_{s}^{+}\to K^{+} K_S^0$
modes. Once the new effect is determined by experiments, the direct $CP$
asymmetry then can be extracted and used to search for new physics.
