Index: /trunk/src/org/openstreetmap/josm/data/UndoRedoHandler.java
===================================================================
--- /trunk/src/org/openstreetmap/josm/data/UndoRedoHandler.java	(revision 2974)
+++ /trunk/src/org/openstreetmap/josm/data/UndoRedoHandler.java	(revision 2975)
@@ -55,5 +55,5 @@
      * Execute the command and add it to the intern command queue.
      */
-    public void add(final Command c) {
+    synchronized public void add(final Command c) {
         addNoRedraw(c);
         afterAdd();
@@ -63,5 +63,5 @@
      * Undoes the last added command.
      */
-    public void undo() {
+    synchronized public void undo() {
         if (commands.isEmpty())
             return;