Index: /applications/editors/josm/plugins/reverter/src/reverter/DataSetCommandMerger.java
===================================================================
--- /applications/editors/josm/plugins/reverter/src/reverter/DataSetCommandMerger.java	(revision 31591)
+++ /applications/editors/josm/plugins/reverter/src/reverter/DataSetCommandMerger.java	(revision 31592)
@@ -5,4 +5,5 @@
 import java.util.ArrayList;
 import java.util.Collection;
+import java.util.LinkedHashSet;
 import java.util.LinkedList;
 import java.util.List;
@@ -97,6 +98,7 @@
         if (!source.isVisible()) return;
         Way target = (Way)getMergeTarget(source);
-        
-        Collection<Conflict<OsmPrimitive>> localConflicts = new ArrayList<>();
+
+        // use a set to avoid conflicts being added twice for closed ways, fixes #11811
+        Collection<Conflict<OsmPrimitive>> localConflicts = new LinkedHashSet<>();
 
         List<Node> newNodes = new ArrayList<>(source.getNodesCount());