search/filter_kanji: keep order when deduplicating

lib/search/filter_kanji.dart

@@ -18,7 +18,15 @@ Future<List<String>> filterKanjiWithDbConnection(
       .then((value) => value.map((e) => e['literal'] as String).toSet());
 
   if (deduplicate) {
-    return filteredKanji.toList();
+    final List<String> result = [];
+    final Set<String> seen = {};
+    for (final k in kanji) {
+      if (filteredKanji.contains(k) && !seen.contains(k)) {
+        result.add(k);
+        seen.add(k);
+      }
+    }
+    return result;
   } else {
     return kanji.where((k) => filteredKanji.contains(k)).toList();
   }