Index: src/wp-admin/includes/schema.php
===================================================================
--- src/wp-admin/includes/schema.php	(revision 33616)
+++ src/wp-admin/includes/schema.php	(working copy)
@@ -194,8 +194,9 @@
   user_status int(11) NOT NULL default '0',
   display_name varchar(250) NOT NULL default '',
   PRIMARY KEY  (ID),
-  KEY user_login_key (user_login),
-  KEY user_nicename (user_nicename)
+  UNIQUE KEY user_login_key (user_login),
+  UNIQUE KEY user_nicename (user_nicename),
+  UNIQUE KEY user_email_key (user_email)
 ) $charset_collate;\n";
 
 	// Multisite users table
@@ -213,8 +214,9 @@
   spam tinyint(2) NOT NULL default '0',
   deleted tinyint(2) NOT NULL default '0',
   PRIMARY KEY  (ID),
-  KEY user_login_key (user_login),
-  KEY user_nicename (user_nicename)
+  UNIQUE KEY user_login_key (user_login),
+  UNIQUE KEY user_nicename (user_nicename),
+  UNIQUE KEY user_email_key (user_email)
 ) $charset_collate;\n";
 
 	// Usermeta.