On the constant-roll inflation with large and small $\eta_H$

We study the seemingly duality between large and small $\eta_H$ for the constant-roll inflation with the second slow-roll parameter $\eta_H$ being a constant. In the previous studies, only the constant-roll inflationary models with small $\eta_H$ are found to be consistent with the observations. The seemingly duality suggests that the constant-roll inflationary models with large $\eta_H$ may be also consistent with the observations. We find that the duality between the constant-roll inflation with large and small $\eta_H$ does not exist because both the background and scalar perturbation evolutions are very different. By fitting the constant-roll inflationary models to the observations, we get $-0.016\le\eta_H\le-0.0078$ at the 95\% C.L if we take $N=60$ for the models with increasing $\epsilon_H$ in which inflation ends when $\epsilon_H=1$, and $3.0135\le \eta_H\le 3.021$ at the 68\% C.L., and $3.0115\le \eta_H\le 3.024$ at the 95\% C.L. for the models with decreasing $\epsilon_H$.