shell-command changing the behavior of my program

Question

Consider the following C code:

#include <stdio.h>
#include <unistd.h>

int main() {
  printf("PID: %d\n", getpid());
  fork();
}

This should output the PID of the process and then fork a clone. When trying from a shell, this behaves in the expected way:

$ ./a.out
PID: 380

However if I execute this from within emacs using shell-command (M-!), the output is now different:

PID: 382
PID: 382

If I modify the program a bit:

#include <stdio.h>
#include <unistd.h>

int main() {
  printf("PID-pre: %d\n", getpid());
  fork();
  printf("PID-post: %d\n", getpid());
}

I can see the proper output in an external terminal:

PID-pre: 398
PID-post: 398
PID-post: 399

However, again the shell-command executes the pre-fork printf twice:

PID-pre: 402
PID-post: 402
PID-pre: 402
PID-post: 403

How come shell-command executes the whole process twice when using fork? Is this the intended behavior? How can I work around it?

My system:

• OS: Fedora 35
• kernel: Linux fedora 5.15.12-200.fc35.x86_64
• lib C: GNU libc 2.34
• emacs: GNU Emacs 27.2 (build 1, x86_64-pc-linux-gnu, GTK+ Version 3.24.30) of 2021-09-25