title: npm scripts using node-sass and autprefixer cause browser-sync to run twice
body: <p>I have the following npm scripts setup to do all my scss and js linting, compiling and bundling. They do work, however It seems there is room for improvement.</p>
  
  <pre><code>"scripts": {
    "lint-scss": "stylelint ./styles/**/*.scss --cache --syntax scss",
    "scss": "node-sass --omit-source-map-url --recursive --output-style compressed --output ./styles ./styles",
    "autoprefixer": "postcss --use autoprefixer --replace ./styles/style.css --no-map",
    "build:css": "yarn run lint-scss --silent | yarn run scss --silent | yarn run autoprefixer --silent",
    "serve": "browser-sync start --https --no-notify --proxy 'project.local' --files './styles/**/*.css, ./views/**/*.php, ./**/*.html, !node_modules/**/*.html'",
    "watch:css": "onchange './styles/**/*.scss' -- yarn run build:css --silent",
    "watch:all": "parallelshell 'yarn run serve --silent' 'yarn run watch:css --silent'",
    "postinstall": "yarn run watch:all --silent"
  }
  </code></pre>
  
  <p>Currently when I watch for changes and save an scss file, browser-sync fires twice. This is due to node-sass running and changing a file then autoprefixer running and changing the same file. (Output below for completeness)</p>
  
  <pre><code>Rendering Complete, saving .css file...
  Wrote CSS to /Users/Matt/Sites/Project/styles/style.css
  Wrote 1 CSS files to /Users/Matt/Sites/Project/styles
  [BS] File event [change] : styles/style.css
  ✔ Finished styles/style.css (192ms)
  [BS] File event [change] : styles/style.css
  </code></pre>
  
  <p>Surely I can merge these and change the file once and have browser-sync fire once?</p>
  
  <p>Thanks</p>