diff --git a/source/net/yacy/kelondro/index/RowCollection.java b/source/net/yacy/kelondro/index/RowCollection.java
index bd467f2b8..f0ae59e56 100644
--- a/source/net/yacy/kelondro/index/RowCollection.java
+++ b/source/net/yacy/kelondro/index/RowCollection.java
@@ -616,8 +616,11 @@ public class RowCollection implements Sortable<Row.Entry>, Iterable<Row.Entry>,
 
     public final void sort() {
         if (this.sortBound == this.chunkcount) return; // this is sorted
-        net.yacy.cora.storage.Array.sort(this);
-        this.sortBound = this.chunkcount;
+        synchronized (this) {
+            if (this.sortBound == this.chunkcount) return; // check again
+            net.yacy.cora.storage.Array.sort(this);
+            this.sortBound = this.chunkcount;
+        }
     }
 
     public static class partitionthread implements Callable<Integer> {