Index: trunk/src/org/openstreetmap/josm/actions/mapmode/ParallelWays.java
===================================================================
--- trunk/src/org/openstreetmap/josm/actions/mapmode/ParallelWays.java	(revision 15556)
+++ trunk/src/org/openstreetmap/josm/actions/mapmode/ParallelWays.java	(revision 15557)
@@ -191,7 +191,13 @@
     private List<Command> makeAddWayAndNodesCommandList() {
         DataSet ds = OsmDataManager.getInstance().getEditDataSet();
+
         List<Command> commands = new ArrayList<>(sortedNodes.size() + ways.size());
+        Set<Node> dupCheck = new HashSet<>();
         for (int i = 0; i < sortedNodes.size() - (isClosedPath() ? 1 : 0); i++) {
-            commands.add(new AddCommand(ds, sortedNodes.get(i)));
+            Node n = sortedNodes.get(i);
+            // don't add the same node twice, see #18386
+            if (dupCheck.add(n)) {
+                commands.add(new AddCommand(ds, n));
+            }
         }
         for (Way w : ways) {