We report the first observation of the doubly Cabibbo-suppressed decay $D^+_sto K^+K^+pi^-$ using 605 fb$^{-1}$ of data collected with the Belle detector at the KEKB asymmetric-energy $e^+e^-$ collider. The branching ratio with respect to its Cabibbo-favored counterpart $mathcal{B}(D^+_sto K^+K^+pi^-)$ / $mathcal{B}(D^+_sto K^+K^-pi^+)$ is (0.229$pm0.028pm$0.012)%, where the first uncertainty is statistical and the second is systematic. We also report a significantly improved measurement of the doubly Cabibbo-suppressed decay $D^+to K^+pi^+pi^-$, with a branching ratio $mathcal{B}(D^+to K^+pi^+pi^-)$ / $mathcal{B}(D^+to K^-pi^+pi^+)$=(0.569$pm0.018pm$0.014)%.
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