Prevent duplicate import

ext/bg/js/database.js

@@ -161,20 +161,22 @@ class Database {
         }
 
         const indexLoaded = (title, version, entities, hasTerms, hasKanji) => {
+            return this.db.dictionaries.where('title').equals(title).count().then(count => {
+                if (count > 0) {
+                    return Promise.reject(`dictionary "${title}" is already imported`);
+                }
+
                 return this.db.dictionaries.add({title, version, hasTerms, hasKanji}).then(() => {
                     this.entities = entities || {};
 
                     const rows = [];
                     for (const name in entities || {}) {
-                    rows.push({
+                        rows.push({name, value: entities[name], dictionary: title});
-                        name,
-                        value: entities[name],
-                        dictionary: title
-                    });
                     }
 
                     return this.db.entities.bulkAdd(rows);
                 });
+            });
         };
 
         const termsLoaded = (title, entries, total, current) => {