title: forcing only one instance of Asyc sub
body: <p>I'd like to trigger a slow-running sub when particular emails arrive, but I don't want to block execution/freeze outlook.</p>
  
  <p>I've written the sub as <code>Public Async Sub...</code> but part of the code would fall over if the sub was called twice in quick succession.</p>
  
  <p>Is there any way to make a queue such that only one copy of my sub is running at any one time, while ensuring that it's still running in the background?</p>
  
  <p>My solution at the moment is to have a public <code>continue as Boolean</code> with a waiting loop beforehand - but this is a bit on the ugly side!</p>