We studied pairing mechanism of the heavily electron doped FeSe (HEDIS) systems, which commonly have one incipient hole band -- a band top below the Fermi level by a finite energy distance $��_b$ -- at $��$ point and ordinary electron bands at $M$ points in Brillouin zone (BZ). We found that the system allows two degenerate superconducting solutions with the exactly same $T_c$ in clean limit: the incipient $s^{\pm}_{he}$-gap ($��_h^{-} \neq 0$, $��_e^{+} \neq 0$) and $s_{ee}^{++}$-gap ($��_h =0$, $��_e^{+} \neq 0$) solutions with different pairing cutoffs, $��_{sf}$ (spin fluctuation energy) and $��_b$, respectively. The $s_{ee}^{++}$-gap solution, in which the system dynamically renormalizes the original pairing cutoff $��_{sf}$ to $��_{phys}=��_b$ ($< ��_{sf}$), therefore actively eliminates the incipient hole band from forming Cooper pairs, but without loss of $T_c$, becomes immune to the impurity pair-breaking. As a result, the HEDIS systems, by dynamically tuning the pairing cutoff and selecting the $s_{ee}^{++}$-pairing state, can always achieve the maximum $T_c$ -- the $T_c$ of the degenerate $s^{\pm}_{he}$ solution in the ideal clean limit -- latent in the original pairing interactions, even in dirty limit.

12 pages, 11 figures; Appendix A is added