Ptrace Linux anti debugging

I'm not convinced that the double ptrace is really stronger than the first one, a simple GDB.py script let you bypass it:

    first = True
    class ptraceBPT(gdb.Breakpoint):
        def stop(self):
            global first
            gdb.execute("return (long int) {}".format("0" if first else "-1"))
            first = False

    ptraceBPT("ptrace")

what would be interesting is an actual use of ptrace, something like:

    int i = 10;
    int read = ptrace(PTRACE_USER_PEEK, &i, sizeof(i));
    if (read != i) printf("ptrace error");

although it still would be hard to write a GDB breakpoint that mimics it: everything ptrace can do, GDB can do it as well :)