title: Inserting text with execCommand causes a duplication issue
body: <p>I'm working on a Firefox extension that inserts Polish characters to any text field using keyboard shortcuts. It's working for all text fields, except for Facebook, Draft.js, and any React contentEditable DIV.</p>
  
  <p>Steps to reproduce:</p>
  
  <ol>
  <li>Load <a href="http://facebook.github.io/draft-js/" rel="nofollow">http://facebook.github.io/draft-js/</a></li>
  <li>Open Chrome Dev Tools</li>
  <li>Select the Draft.js input box so that the input caret is present </li>
  <li>execute <code>document.execCommand("insertHTML", false, "X");</code> in the dev tools console</li>
  </ol>
  
  <p>Result expected:
  X is added to the input box</p>
  
  <p>Actual result:
  X is added to the input box twice. You're able to delete one of the instances, but the other one is undeletable but is selectable.</p>
  
  <p>Note: Once at least one character has been typed into the Draft.js input the command works as expected.</p>