Index: src/runtime/class.m
==================================================================
--- src/runtime/class.m
+++ src/runtime/class.m
@@ -341,10 +341,17 @@
 		return;
 
 	if (class->superclass)
 		initializeClass(class->superclass);
 
+	/*
+	 * Avoid double-initialization: One of the superclasses' +[initialize]
+	 * might have called this class and hence it already got initialized.
+	 */
+	if (class->info & OBJC_CLASS_INFO_INITIALIZED)
+		return;
+
 	class->info |= OBJC_CLASS_INFO_DTABLE;
 	class->isa->info |= OBJC_CLASS_INFO_DTABLE;
 
 	objc_update_dtable(class);
 	objc_update_dtable(class->isa);