Windows has it already, since many moons ago.
See ntos\mm\forksup.c in the Windows kernel source: "This module contains the routines which support the POSIX fork operation."
Though as someone else mentioned already, it's a poor fit performance-wise for the Windows way of doing process-y memory management-y things.
Reusing the address space of an existing process is definitely supported however, though not exposed by the Win32 API AFAIK.
Biting the hand that feeds IT
