Fitting subgroup is normal-isomorph-free in finite

Statement
In a finite group, the Fitting subgroup is normal-isomorph-free: there is no other normal subgroup of the whole group isomorphic to it.

Related facts

 * Perfect core is homomorph-containing
 * Solvable core is normal-isomorph-free in finite
 * Fitting subgroup not is isomorph-free in finite

Proof idea
In a finite group, the Fitting subgroup is the unique largest nilpotent normal subgroup. Any normal subgroup isomorphic to it would also be a nilpotent normal subgroup, and hence, in particular, would be contained in it. Since the group is finite, this forces that any normal subgroup isomorphic to the Fitting subgroup must be equal to it.