Index: /trunk/src/org/openstreetmap/josm/gui/dialogs/properties/PropertiesDialog.java
===================================================================
--- /trunk/src/org/openstreetmap/josm/gui/dialogs/properties/PropertiesDialog.java	(revision 4139)
+++ /trunk/src/org/openstreetmap/josm/gui/dialogs/properties/PropertiesDialog.java	(revision 4140)
@@ -310,4 +310,23 @@
             Main.main.undoRedo.add(new ChangePropertyCommand(sel, newkey, value));
         } else {
+            for (OsmPrimitive osm: sel) {
+                if(osm.get(newkey) != null) {
+                    ExtendedDialog ed = new ExtendedDialog(
+                            Main.parent,
+                            tr("Overwrite key"),
+                            new String[]{tr("Replace"), tr("Cancel")});
+                    ed.setButtonIcons(new String[]{"purge", "cancel"});
+                    ed.setContent(tr("You changed the key from ''{0}'' to ''{1}''.\n"
+                    + "The new key is already used, overwrite values?", key, newkey));
+                    ed.setCancelButton(2);
+                    ed.toggleEnable("overwriteEditKey");
+                    ed.showDialog();
+
+                    if (ed.getValue() != 1) {
+                        return;
+                    }
+                    break;
+                }
+            }
             Collection<Command> commands=new Vector<Command>();
             commands.add(new ChangePropertyCommand(sel, key, null));
@@ -1258,5 +1277,3 @@
         }
     }
-
-
 }