title: How to know if browser tab is already open using Javascript?
body: <p>How to know or check if the two browser tab is already open and if those tab are open, the user will receive an alert box or msg box saying that 'the url is already open', something like that, in pure/native JavaScript? This browser tab is contain an external website which is I don't have any privileges to manipulate or change it. Thanks</p>
  
  <p>Example URLs</p>
  
  <pre><code>yahoo.com and google.com
  </code></pre>
  
  <p>I want to alert the user if there's already open tab for yahoo.com and google.com</p>
  
  <p>And I want to use tabCreate to open the url like this:</p>
  
  <pre><code>tabCreate("http://maps.google.com/", "tabMapsPermanentAddress");
  mean to open a new tab, it is use in creating chrome extension
  </code></pre>