diff --git a/src/main/java/de/ids_mannheim/korap/query/serialize/QuerySerializer.java b/src/main/java/de/ids_mannheim/korap/query/serialize/QuerySerializer.java
index 643eb08..c8f6c56 100644
--- a/src/main/java/de/ids_mannheim/korap/query/serialize/QuerySerializer.java
+++ b/src/main/java/de/ids_mannheim/korap/query/serialize/QuerySerializer.java
@@ -178,8 +178,8 @@
             List errors = (List) requestMap.get("errors");
             List warnings = (List) requestMap.get("warnings");
             List messages = (List) requestMap.get("messages");
-            this.collection = mergeCollection(collection, this.collection);
-            requestMap.put("collection", this.collection);
+            collection = mergeCollection(collection, this.collection);
+            requestMap.put("collection", collection);
             if (this.meta != null) {
                 this.meta.putAll(meta);
                 requestMap.put("meta", this.meta);
@@ -226,6 +226,8 @@
             return collection2;
         }else if (collection2 == null || collection2.isEmpty()) {
             return collection1;
+        }else if (collection1.equals(collection2)) {
+            return collection1;
         }else {
             LinkedHashMap<String, Object> docGroup = KoralObjectGenerator
                     .makeDocGroup("and");