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Type Ic supernovae (SNe Ic) arise from the core-collapse of H (and He) poorstars, which could be either single WR stars or lower-mass stars stripped oftheir envelope by a companion. Their light curves are radioactively powered andusually show a fast rise to peak ($\sim$10-15 d), without any early (first fewdays) emission bumps (with the exception of broad-lined SNe Ic) as sometimesseen for other types of stripped-envelope SNe (e.g., Type IIb SN 1993J and TypeIb SN 2008D). We have studied iPTF15dtg, a spectroscopically normal SN Ic withan early excess in the optical light curves followed by a long ($\sim$30 d)rise to the main peak. It is the first spectroscopically-normal double-peakedSN Ic observed. We aim to determine the properties of this explosion and of itsprogenitor star. Optical photometry and spectroscopy of iPTF15dtg was obtainedwith multiple telescopes. The resulting light curves and spectral sequence areanalyzed and modelled with hydrodynamical and analytical models, withparticular focus on the early emission. Results. iPTF15dtg is a slow rising SNIc, similar to SN 2011bm. Hydrodynamical modelling of the bolometric propertiesreveals a large ejecta mass ($\sim$10 $M_{\odot}$) and strong $^{56}$Ni mixing.The luminous early emission can be reproduced if we account for the presence ofan extended ($\sim$500 R$_{\odot}$), low-mass ($\sim$0.045 M$_{\odot}$)envelope around the progenitor star. Alternative scenarios for the early peak,such as the interaction with a companion, a shock-breakout (SBO) cooling tailfrom the progenitor surface, or a magnetar-driven SBO are not favored. Thelarge ejecta mass and the presence of H and He free extended material aroundthe star suggest that the progenitor of iPTF15dtg was a massive ($\gtrsim$ 35M$_{\odot}$) WR star suffering strong mass loss.

iPTF15dtg: a double-peaked Type Ic Supernova from a massive progenitor