--- src/main/java/hudson/remoting/RemoteClassLoader.java	2012-06-07 16:24:34.093017000 +0200
+++ src/main/java/hudson/remoting/RemoteClassLoader.java	2012-06-07 16:47:38.350454492 +0200
@@ -126,9 +126,13 @@
                 ClassLoader cl = channel.importedClassLoaders.get(cf.classLoader);
                 if (cl instanceof RemoteClassLoader) {
                     RemoteClassLoader rcl = (RemoteClassLoader) cl;
-                    Class<?> c = rcl.findLoadedClass(name);
-                    if (c==null)
-                        c = rcl.loadClassFile(name,cf.classImage);
+                	Class<?> c;
+                    synchronized (this) {
+                    	c = rcl.findLoadedClass(name);
+                    
+                    	if (c==null)
+                    		c = rcl.loadClassFile(name,cf.classImage);
+                    }
                     return c;
                 } else {
                     return cl.loadClass(name);