  There is a duality in the observables $n_s$, $r$ and the inflaton potential
between large and small $\eta_H$ for the constant-roll inflation if the
slow-roll parameter $\epsilon_H$ is negligible. In general, the duality between
$\eta_H$ and $\bar{\eta}_H$ does not hold for the background evolution of the
inflation. For some particular solutions for the constant-roll inflation with
$\eta_H$ being a constant, we find that in the small field approximation, the
potential takes the quadratic form and it remains the same when the parameter
$\eta_H$ changes to $\bar{\eta}_H=3-\eta_H$. If the scalar field is small and
the contribution of $\epsilon_H$ is negligible, we find that there exists the
logarithmic duality and the duality between large and small $\eta_H$ for the
primordial curvature perturbation in inflationary models with the quadratic
potential.
