  In a recent Physical Review Letter Kortus et al. claim that the observed
decrease of $T_c$ in Al and C doped MgB$_2$ samples is mainly due to the band
filling by the electron doping but for a fuller understanding of the different
behavior of the superconducting energy gaps in Al and C doped samples it is
necessary to also include an increased interband scattering for the carbon
doped samples. We argue in the following that this latter statement is
misleading and is based on the assumption that for carbon doping the two
superconducting gaps merge near 10 % carbon doping: an assumption that
contradicts most of the existing experimental data.

Superconductivity