title: VScode extension clean diagnostic collection on second run
body: <p>I am creating a <code>diagnosticCollection</code> on a vscode extension following this 
  <a href="https://github.com/microsoft/vscode-extension-samples/blob/master/diagnostic-related-information-sample/src/extension.ts" rel="nofollow noreferrer">sample:</a></p>
  
  <p>It works but if I run the same command twice on the same file it creates a "duplicate" diagnostics issues list in the problems terminal window.</p>
  
  <p>What is the best way to clear that terminal window problems? I have tried to run <code>collections.clear()</code> which I thought would do the job but it did not work.</p>