  We report the first detection in space of a complete sample of nine doubly
substituted isotopologues of HCCCN towards the cyanopolyyne peak of TMC-1 using
observations of the QUIJOTE line survey taken with the Yebes 40 m telescope. We
detected D13CCCN, DC13CCN, DCC13CN, DCCC15N, H13C13CCN, H13CC13CN, HC13C13CN,
HCC13C15N, and HC13CC15N through their J=4-3 and J=5-4 lines in the 7 mm
window. In addition, we present an extensive analysis of the emission of HCCCN
and its singly substituted isotopologues through a large velocity gradient
model of the lines detected at 7 mm and 3 mm using the Yebes 40 m and the IRAM
30 m telescopes, respectively. The derived column densities for all the
isotopologues are consistent in the two spectral bands for an H2 volume density
of 1e4 cm-3 and a kinetic temperature of 10 K. Whereas we observed a 13C
fractionation for HCC13CN and other double isotopologues with a 13C atom
adjacent to the nitrogen atom, we derived similar C/13C abundance ratios for
the three 13C substituted species of DCCCN. This suggests additional chemical
discrimination for deuterated isotopologues of HCCCN. Finally, we present the
spatial distribution of the J=4-3 and J=5-4 lines from the singly substituted
species observed with the Yebes 40 m telescope. The emission peak of the
spatial distribution of DCCCN appears to be displaced by 40'' with respect to
that of HCCCN and the 13C and 15N isotopologues. In addition to a different
formation route for the deuterated species, we could also expect that this
differentiation owing to the deuterium fractionation is more efficient at low
temperatures, and therefore, that deuterated species trace a colder region of
the cloud.
