Strange Thread Causing Duplicating `writeln`

[Cym13]

On Monday, 9 April 2018 at 22:28:45 UTC, Jonathan wrote:
> I am totally lost on why this is happening.
>
> I stripped the code down to what appears to be the most minimal 
> code that still causes the problem.
>
> ---
> import core.sync.mutex;
> import core.thread;
> import std.stdio;
>
> __gshared Mutex m;//__gshared just for testing (;
>
> void thread1() {
> 	foreach (i;0..8) {
> 		synchronized(m) {
> 			writeln("a1-",i);
> 		}
> 		writeln("a2-",i);
> 	}
> }
> void thread2() {
> 	foreach (i;0..8) {
> 		synchronized(m) {
> 			writeln("b1-",i);
> 		}
> 		writeln("b2-",i);
> 	}
> }
>
>
> void main() {
> 	m = new Mutex();
> 	
> 	new Thread(&thread1).start;
> 	new Thread(&thread2).start;
> }
> ---
> The beginning of the output for this code is:
> a1-0
> a2-0
> a2-0
> b1-0
> b2-0
> b2-0
> a1-1
> a2-1
> a2-1
>
> Why is the "a2" and "b2" writeln being repeated?!

I don't know, but I can't reproduce either with dmd or ldc. What 
was your compilation line?