title: Chrome DevTools extension loads twice when opened in different tabs
body: <p>I'm building a Chrome extension in DevTools. It works great unless I open it in another tab. When it is opened in another tab it loads my inserted-script.js file for each instance that the DevTools extension is open. What might be causing this? I'm happy to share what I can, but I'm unsure where to start.</p>
  
  <p>Edit: Here's the manifest.json file:</p>
  
  <pre><code>{
      "name": "My DevTools Thing",
      "version": "0.5",
      "description": "Hello.",
      "devtools_page": "devtools.html",
      "background": {
          "scripts": [
              "js/background.js"
          ]
      },
      "permissions": [
          "activeTab",
          "http://*/*",
          "https://*/*"
      ],
      "web_accessible_resources": [
          "js/content.js"
      ],
      "manifest_version": 2
  }
  </code></pre>
  
  <p>Edit 2: To add some further context - let's say I have devTools open on tab 1 and all it does it display "Hello!". The first time it's open it displays once.</p>
  
  <p>I then open a new tab, open devTools, then my extension, it shows:</p>
  
  <p>"Hello!"</p>
  
  <p>"Hello!"</p>
  
  <p>On my original tab it has now appended:</p>
  
  <p>"Hello!" (the original)</p>
  
  <p>"Hello!" (from the new tab)</p>
  
  <p>"Hello!" (from the new tab)</p>
  
  <p>If I open a THIRD tab and open the extension...</p>
  
  <p>"Hello!" x 3 (new tab)</p>
  
  <p>The other tabs will have the 3 "Hello!"s from the original appended to it.</p>