  Type Ia supernovae (SNe Ia) play key roles in revealing the accelerating
expansion of the universe, but our knowledge about their progenitors is still
very limited. Here we report the discovery of a rigid dichotomy in
circumstellar (CS) environments around two subclasses of type Ia supernovae
(SNe Ia) as defined by their distinct photospheric velocities. For the SNe Ia
with high photospheric velocities (HV), we found a significant excess flux in
blue light during 60-100 days past maximum, while this phenomenon is absent for
SNe with normal photospheric velocity (Normal). This blue excess can be
attributed to light echoes by circumstellar dust located at a distance of about
1-3x10^{17} cm from the HV subclass. Moreover, we also found that the HV SNe Ia
show systematically evolving Na I absorption line by performing a systematic
search of variable Na I absorption lines in spectra of all SNe Ia, whereas this
evolution is rarely seen in Normal ones. The evolving Na I absorption can be
modeled in terms of photoionization model, with the location of the gas clouds
at a distance of about 2x10^{17} cm, in striking agreement with the location of
CS dust inferred from B-band light curve excess. These observations show
clearly that the progenitors of HV and Normal subclasses are systematically
different, suggesting that they are likely from single and double degenerate
progenitor systems, respectively.
