Index: wp-admin/includes/upgrade.php
===================================================================
--- wp-admin/includes/upgrade.php	(revision 15676)
+++ wp-admin/includes/upgrade.php	(working copy)
@@ -1956,7 +1956,8 @@
   activation_key varchar(50) NOT NULL default '',
   meta longtext,
   KEY activation_key (activation_key),
-  KEY domain (domain)
+  KEY domain (domain),
+  KEY domain (user_email)
 ) $charset_collate;
 ";
 // now create tables