SN 2013aa and SN 2017cbv: Two Sibling Type Ia Supernovae in the spiral
  galaxy NGC 5643
  We present photometric and spectroscopic observations of SN 2013aa and SN
2017cbv, two nearly identical type Ia supernovae (SNe Ia) in the host galaxy
NGC 5643. The optical photometry has been obtained using the same telescope and
instruments used by the Carnegie Supernova Project. This eliminates most
instrumental systematics and provides light curves in a stable and
well-understood photometric system. Having the same host galaxy also eliminates
systematics due to distance and peculiar velocity, providing an opportunity to
directly test the relative precision of SNe Ia as standard candles. The two SNe
have nearly identical decline rates, negligible reddening, and remarkably
similar spectra and, at a distance of $\sim 20$ Mpc, are ideal as potential
calibrators for the absolute distance using primary indicators such as Cepheid
variables. We discuss to what extent these two SNe can be considered twins and
compare them with other supernova "siblings" in the literature and their likely
progenitor scenarios. Using 12 galaxies that hosted 2 or more SNe~Ia, we find
that when using SNe~Ia, and after accounting for all sources of observational
error, one gets consistency in distance to 3 percent.
