いきなりですが修正はいりました。

おはずかしいかぎりです。

• https://github.com/kiririmode/AlreadyHatenaBookmarked

$ git diff HEAD~2 HEAD
diff --git a/alreadyhatenabookmarked.user.js b/alreadyhatenabookmarked.user.js
index 07247aa..bb797ee 100644
--- a/alreadyhatenabookmarked.user.js
+++ b/alreadyhatenabookmarked.user.js
@@ -19,7 +19,7 @@

     function init() {
         addGlobalStyle([
-            'div#' + CSS_DIV_ID  + ' { bottom: 20px; right: 0px; position: fixed; width: 300px; height: 20px; background: #00CED1; z-index: 1000 }',
+            'div#' + CSS_DIV_ID  + ' { bottom: 20px; right: 0px; position: fixed; width: 300px; height: 30px; background: #00CED1; z-index: 1000 }',
             'div#' + CSS_TEXT_ID + ' { color: black }'
         ].join("\n"));

@@ -68,7 +68,7 @@
     }

     function display(responseText){
-        if ( responseText == null ) {
+        if ( responseText == 'null' ) {
             createDisplayBox("not bookmarked");
             return;
         }
@@ -80,7 +80,7 @@
                 continue;
             }

-            var msg = bookmarks[idx].timestamp + " already bookmarked: \n" +  bookmarks[idx].tags;
+            var msg = bookmarks[idx].timestamp + " already bookmarked: \n" + ((bookmarks[idx].tags)? bookmarks[idx].tags.join(" ") : "");
             createDisplayBox(msg);
             return;
         }