code: function dedupeCss() {
    const prefixRegex = /-(webkit|moz|ms|o)-.+/;
  
    return insert.transform(function(contents) {
      // Parse the CSS into an AST.
      const parsed = postcss.parse(contents);
  
      // Walk through all the rules, skipping comments, media queries etc.
      parsed.walk(function(rule) {
        // Skip over any comments, media queries and rules that have less than 2 properties.
        if (rule.type !== 'rule' || !rule.nodes || rule.nodes.length < 2) return;
  
        // Walk all of the properties within a rule.
        rule.walk(function(prop) {
          // Check if there's a similar property that comes after the current one.
          const hasDuplicate = validateProp(prop) && _.find(rule.nodes, function(otherProp) {
            return prop !== otherProp && prop.prop === otherProp.prop && validateProp(otherProp);
          });
  
          // Remove the declaration if it's duplicated.
          if (hasDuplicate) {
            prop.remove();
  
            gutil.log(gutil.colors.yellow(
              'Removed duplicate property: "' +
              prop.prop + ': ' + prop.value + '" from "' + rule.selector + '"...'
            ));
          }
        });
      });
  
      // Turn the AST back into CSS.
      return parsed.toResult().css;
    });
  
    // Checks if a property is a style declaration and that it
    // doesn't contain any vendor prefixes.
    function validateProp(prop) {
      return prop && prop.type === 'decl' && ![prop.prop, prop.value].some(function(value) {
        return value.indexOf('-') > -1 && prefixRegex.test(value);
      });
    }
  }
docs: Removes duplicated CSS properties.