dup() is per filedes, not per fd

dup.c

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

int main(void)
{
  int fd = dup(0);
  printf("O_NONBLOCK=%d\n", O_NONBLOCK & fcntl(0, F_GETFL));
  fcntl(fd, F_SETFL, O_NONBLOCK | fcntl(fd, F_GETFL));
  printf("O_NONBLOCK=%d\n", O_NONBLOCK & fcntl(0, F_GETFL));
  return 0;
}