Index: trunk/src/org/openstreetmap/josm/gui/layer/LayerManager.java
===================================================================
--- trunk/src/org/openstreetmap/josm/gui/layer/LayerManager.java	(revision 11296)
+++ trunk/src/org/openstreetmap/josm/gui/layer/LayerManager.java	(revision 11297)
@@ -192,5 +192,5 @@
     protected synchronized void realAddLayer(Layer layer) {
         if (containsLayer(layer)) {
-            throw new IllegalArgumentException("Cannot add a layer twice.");
+            throw new IllegalArgumentException("Cannot add a layer twice: " + layer);
         }
         LayerPositionStrategy positionStrategy = layer.getDefaultLayerPosition();