make ssr style injection idempotent

Author: yyx990803

lib/addStylesServer.js

@@ -38,8 +38,10 @@ function addStyleProd (styles, list) {
       var id = part.media || 'default'
       var style = styles[id]
       if (style) {
-        style.ids.push(part.id)
-        style.css += '\n' + part.css
+        if (style.ids.indexOf(part.id) < 0) {
+          style.ids.push(part.id)
+          style.css += '\n' + part.css
+        }
       } else {
         styles[id] = {
           ids: [part.id],