Index: /trunk/src/org/openstreetmap/josm/gui/layer/OsmDataLayer.java
===================================================================
--- /trunk/src/org/openstreetmap/josm/gui/layer/OsmDataLayer.java	(revision 1557)
+++ /trunk/src/org/openstreetmap/josm/gui/layer/OsmDataLayer.java	(revision 1558)
@@ -19,4 +19,5 @@
 import java.awt.event.ActionEvent;
 import java.awt.geom.Area;
+import java.awt.geom.Rectangle2D;
 import java.awt.image.BufferedImage;
 import java.io.File;
@@ -234,7 +235,13 @@
 //        System.out.println("");
 
-        // copy the merged layer's data source info
-        for (DataSource src : ((OsmDataLayer)from).data.dataSources)
-            data.dataSources.add(src);
+        Area a = data.getDataSourceArea();
+        
+        // copy the merged layer's data source info; 
+        // only add source rectangles if they are not contained in the
+        // layer already.
+        for (DataSource src : ((OsmDataLayer)from).data.dataSources) {
+            if (a == null || !a.contains(src.bounds.asRect()))
+                data.dataSources.add(src);
+        }
         
         // copy the merged layer's API version, downgrade if required