While there is a broad consensus that Type Ia supernovae (SNe Ia) arise from explosions of carbon/oxygen (C/O) white dwarfs (WDs), the traditional scenario involving a Chandrasekhar-mass WD cannot account for the entire population. The detonation of a thin (∼0.01M_sun) helium shell atop a ∼1M_sun C/O core is a promising mechanism to explode sub-Chandrasekhar-mass WDs. These double-detonation explosions may be the origin of many normal SNe Ia. More massive helium shells and/or less massive C/O cores may explain some recently observed peculiar objects. In this talk, I will present observations of two SNe Ia discovered by Zwicky Transient Facility (ZTF), SNe 2020jgb and 2022joj. Their remarkable peculiarities point toward a double-detonation origin - (i) unusual color evolution and prominent continuous absorption in near-ultra-violet probe Fe-group elements formed in helium-shell detonations; (ii) tentative features of unburnt helium is detected in the near-infrared spectrum of SN 2020jgb; (iii) the nebular-phase spectra of SN 2022joj indicate a low Ni/Fe abundance ratio in the SN ejecta, which is only expected in the explosion of a sub-Chandrasekhar-mass WD. To close, I will briefly introduce the La Silla Schmidt Southern Survey (LS4), a new wide-field time-domain survey. With a customized color-evolution filter in the transient alert stream, LS4 will naturally power the discovery of both normal and peculiar double-detonation SNe Ia in a prompt and systematic way.