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.
Index: src/wp-admin/includes/upgrade.php
===================================================================
--- src/wp-admin/includes/upgrade.php	(revision 33616)
+++ src/wp-admin/includes/upgrade.php	(working copy)
@@ -2590,6 +2590,16 @@
 		$wpdb->query( "ALTER TABLE $wpdb->postmeta DROP INDEX meta_key, ADD INDEX meta_key(meta_key(191))" );
 		$wpdb->query( "ALTER TABLE $wpdb->posts DROP INDEX post_name, ADD INDEX post_name(post_name(191))" );
 	}
+
+	// Upgrade versions prior to 4.4.
+	if ( $wp_current_db_version < 31415 && wp_should_upgrade_global_tables() ) {
+		$users = $wpdb->get_results( "SELECT ID, user_login, user_email, COUNT(*) FROM $wpdb->users GROUP BY user_email HAVING COUNT(*) > 1" );
+		foreach( $users as $user ){
+			$email = $user->user_login . '+' . $user->user_email;
+			$wpdb->update( $wpdb->users ,array( 'user_email' => $email ), array( 'ID' => $user->ID ),'%s','%d' );
+		}
+		$wpdb->query( "ALTER TABLE $wpdb->users ADD ( CONSTRAINT user_login_key UNIQUE (user_login), CONSTRAINT user_nicename UNIQUE (user_nicename),  CONSTRAINT user_email_key UNIQUE (user_email) )" );
+	}
 }
 
 /**