--- a/hotspot/src/share/vm/classfile/modules.cpp	Tue Apr 19 14:53:33 2016 +0200
+++ b/hotspot/src/share/vm/classfile/modules.cpp	Thu Apr 21 09:23:04 2016 -0400
@@ -277,6 +277,11 @@
   {
     MutexLocker m1(Module_lock, THREAD);
 
+    if (ModuleEntryTable::javabase_defined()) {
+      THROW_MSG(vmSymbols::java_lang_IllegalArgumentException(),
+                "Module java.base is already defined");
+    }
+
     // Verify that all java.base packages created during bootstrapping are in
     // pkg_list.  If any are not in pkg_list, than a non-java.base class was
     // loaded erroneously pre java.base module definition.