Index: trunk/src/org/openstreetmap/josm/data/osm/visitor/MergeVisitor.java
===================================================================
--- trunk/src/org/openstreetmap/josm/data/osm/visitor/MergeVisitor.java	(revision 2405)
+++ trunk/src/org/openstreetmap/josm/data/osm/visitor/MergeVisitor.java	(revision 2406)
@@ -135,4 +135,8 @@
         // my dataset. Just add other to my dataset.
         //
+
+        //TODO primitive must be only in one dataset at one time, so remove it from theirDataset. This obviously
+        // breaks theirDataset and we can only hope that nobody will try to use theirDataset after merging is finished
+        theirDataSet.removePrimitive(other);
         myDataset.addPrimitive(other);
     }