diff --git a/background/usercss-helper.js b/background/usercss-helper.js
index f1c6419c..546f1172 100644
--- a/background/usercss-helper.js
+++ b/background/usercss-helper.js
@@ -75,6 +75,7 @@
    * @returns {Promise<{style, dup:Boolean?}>}
    */
   function build({
+    styleId,
     sourceCode,
     checkDup,
     metaOnly,
@@ -83,7 +84,8 @@
   }) {
     return usercss.buildMeta(sourceCode)
       .then(style => {
-        const findDup = checkDup || assignVars ? find(style) : null;
+        const findDup = checkDup || assignVars ?
+          find(styleId ? {id: styleId} : style) : Promise.resolve();
         return Promise.all([
           metaOnly ? style : doBuild(style, findDup),
           findDup
diff --git a/edit/source-editor.js b/edit/source-editor.js
index dcd43a13..59209c5b 100644
--- a/edit/source-editor.js
+++ b/edit/source-editor.js
@@ -82,6 +82,7 @@ function createSourceEditor(style) {
 
   function preprocess(style) {
     return API.buildUsercss({
+      styleId: style.id,
       sourceCode: style.sourceCode,
       assignVars: true
     })