  In this paper the massless NJL model extended by the diquark interaction
channel is considered. We study its phase structure at zero temperature and in
the presence of baryon $\mu_B$, isospin $\mu_I$, chiral $\mu_{5}$ and chiral
isospin $\mu_{I5}$ chemical potentials in the mean-field approximation. It is
shown that the model thermodynamic potential, which depends on three order
parameters, $M$, $\pi_1$ and $\Delta$ (where $M$, $\pi_1$, and $\Delta$ -- are,
respectively, chiral, charged pion and diquark condensates of the model), is
symmetric with respect to the (dual) transformation when
$M\leftrightarrow\pi_1$ and simultaneously $\mu_I\leftrightarrow\mu_{I5}$. As a
result, on the mean-field phase portrait of the model the chiral symmetry
breaking (CSB) and charged pion condensation (PC) phases turn out to be dually
conjugate with each other, which greatly simplifies the study of the phase
portrait of the model. In particular, the duality between CSB and charged PC
phases means that in the $(\mu_I,\mu_{I5})$-phase portrait these phases are
mirror-symmetrical with respect to the line $\mu_I=\mu_{I5}$, which at the same
time is the symmetry axis of the color superconducting (CSC) phase. Moreover,
it follows from our analysis that chiral $\mu_5$ chemical potential promotes
the formation of CSC phase in dense quark matter. And together with $\mu_{I5}$
it can generates the charged PC phase even at $\mu_I=0$.
