We present a detailed analysis of GRB 151006A, the first GRB detected by Astrosat CZT Imager (CZTI). We study the long term spectral evolution by exploiting the capabilities of emph{Fermi} and emph{Swift} satellites at different phases, which is complemented by the polarization measurement with the CZTI. While the light curve of the GRB in different energy bands show a simple pulse profile, the spectrum shows an unusual evolution. The first phase exhibits a hard-to-soft (HTS) evolution until $sim16-20$,s, followed by a sudden increase in the spectral peak reaching a few MeV. Such a dramatic change in the spectral evolution in case of a single pulse burst is reported for the first time. This is captured by all models we used namely, Band function, Blackbody+Band and two blackbodies+power law. Interestingly, the emph{Fermi} Large Area Telescope (LAT) also detects its first photon ($>100$,MeV) during this time. This new injection of energy may be associated with either the beginning of afterglow phase, or a second hard pulse of the prompt emission itself which, however, is not seen in the otherwise smooth pulse profile. By constructing Bayesian blocks and studying the hardness evolution we find a good evidence for a second hard pulse. The emph{Swift} data at late epochs ($>T_{90}$ of the GRB) also shows a significant spectral evolution consistent with the early second phase. The CZTI data (100--350,keV), though having low significance ($1sigma$), show high values of polarization in the two epochs ($77%$ to $94%$), in agreement with our interpretation.
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