Index: src/org/openstreetmap/josm/plugins/JunctionChecker/JunctionCheckerMapMode.java
===================================================================
--- src/org/openstreetmap/josm/plugins/JunctionChecker/JunctionCheckerMapMode.java	(revision 34950)
+++ src/org/openstreetmap/josm/plugins/JunctionChecker/JunctionCheckerMapMode.java	(working copy)
@@ -41,7 +41,11 @@
         super.enterMode();
         MainApplication.getMap().mapView.addMouseListener(this);
         MainApplication.getMap().mapView.addMouseMotionListener(this);
-        MainApplication.getLayerManager().addActiveLayerChangeListener(this);
+        try {
+        	MainApplication.getLayerManager().addActiveLayerChangeListener(this);
+        } catch (IllegalArgumentException e) {
+        	// This will (hopefully) only occur when this has already been added...
+        }
     }
 
     @Override public void exitMode() {