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In this letter, a novel double-pass in-line fiber taper Mach-Zehnder interferometer (MZI) is proposed and demonstrated as a refractive index and temperature sensor. The double-pass configuration is achieved by connecting a reflecting mirror to one port of the MZI. The attenuation peaks of the double-pass MZI spectra have higher contrasts than the peaks of single-pass spectra. Using our recently proposed spectrum differential integration analyzing method, the double-pass MZI sensors' sensitivities can be made nearly twice as large as those of the single-pass MZI.