title: What would be the behaviour of GmailApp.createLabel when called multiple times with same label name?
body: <p>I tried the following code.</p>
  
  <pre><code>function addLabel() {
    console.log(GmailApp.createLabel('FOO'));
    console.log(GmailApp.createLabel('FOO'));
  }
  </code></pre>
  
  <p>After running this function, I see that there is only one label <code>FOO</code> and the threads which are assigned earlier to <code>FOO</code> are retained. And no exception thrown at runtime for 'duplicate label name'.</p>
  
  <p>Is this a valid behaviour? Can it be relied upon? The Official <a href="https://developers.google.com/apps-script/reference/gmail/gmail-app#createlabelname" rel="nofollow noreferrer">documentation</a> doesn't mention anything like this. </p>