As admin, enable "Mobile services": Site administration ► Mobile app ► Mobile settings Setup a service in Moodle (Site Administration > Plugins > Webservices > External Service), adding the core_user_update_users WS. Create a token for an admin user (Site Administration > Plugins > Webservices > Manage protocols) for the service Disable the allowaccountssameemail global site setting Using this PHP client try to update an existing user account using an email address already in use in the site (by a different user). In the php client, you will need to use the core_user_update_users WS and remove all the $user1 and $user2 data except $user1->id and $user1->email Check that after running the WS, the email is not updated in the user profile. Enable the allowaccountssameemail global site setting Run again the WS client and check that this time the email is successfully updated and you have two users in the site using the same email address