diff --git a/auth/oauth2/classes/api.php b/auth/oauth2/classes/api.php
index 430abeef8d3..ffcb79c47fb 100644
--- a/auth/oauth2/classes/api.php
+++ b/auth/oauth2/classes/api.php
@@ -105,6 +105,10 @@ class api {
             $userid = $USER->id;
         }
 
+        if (linked_login::count_records(['username' => $userinfo['username']]) > 0) {
+            throw new moodle_exception('alreadylinked', 'auth_oauth2');
+        }
+
         if (\core\session\manager::is_loggedinas()) {
             throw new moodle_exception('notwhileloggedinas', 'auth_oauth2');
         }
@@ -144,9 +148,8 @@ class api {
         $record->issuerid = $issuer->get('id');
         $record->username = $userinfo['username'];
         $record->userid = $userid;
-        $existing = linked_login::get_record((array)$record);
-        if ($existing) {
-            return false;
+        if (linked_login::count_records(['username' => $userinfo['username']]) > 0) {
+            throw new moodle_exception('alreadylinked', 'auth_oauth2');
         }
         $record->email = $userinfo['email'];
         $record->confirmtoken = random_string(32);
@@ -239,6 +242,10 @@ class api {
         require_once($CFG->dirroot.'/user/profile/lib.php');
         require_once($CFG->dirroot.'/user/lib.php');
 
+        if (linked_login::count_records(['username' => $userinfo['username']]) > 0) {
+            throw new moodle_exception('alreadylinked', 'auth_oauth2');
+        }
+
         $user = new stdClass();
         $user->username = $userinfo['username'];
         $user->email = $userinfo['email'];
@@ -319,4 +326,18 @@ class api {
 
         $login->delete();
     }
+
+    /**
+     * Delete linked logins for a user.
+     *
+     * @param \core\event\user_deleted $event
+     * @return boolean
+     */
+    public static function user_deleted(\core\event\user_deleted $event) {
+        global $DB;
+
+        $userid = $event->objectid;
+
+        return $DB->delete_records(linked_login::TABLE, ['userid' => $userid]);
+    }
 }
diff --git a/auth/oauth2/db/events.php b/auth/oauth2/db/events.php
new file mode 100644
index 00000000000..b6f793c38d1
--- /dev/null
+++ b/auth/oauth2/db/events.php
@@ -0,0 +1,31 @@
+<?php
+// This file is part of Moodle - http://moodle.org/
+//
+// Moodle is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// Moodle is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with Moodle.  If not, see <http://www.gnu.org/licenses/>.
+
+/**
+ * This file definies observers needed by the plugin.
+ *
+ * @package    auth_oauth2
+ * @copyright  2017 Damyon Wiese
+ * @license    http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
+ */
+
+// List of observers.
+$observers = [
+    [
+        'eventname'   => '\core\event\user_deleted',
+        'callback'    => '\auth_oauth2\api::user_deleted',
+    ],
+];
diff --git a/auth/oauth2/lang/en/auth_oauth2.php b/auth/oauth2/lang/en/auth_oauth2.php
index f0715e0a489..1ba808c28b5 100644
--- a/auth/oauth2/lang/en/auth_oauth2.php
+++ b/auth/oauth2/lang/en/auth_oauth2.php
@@ -83,3 +83,4 @@ $string['notwhileloggedinas'] = 'Linked logins cannot be managed while logged in
 $string['oauth2:managelinkedlogins'] = 'Manage own linked login accounts';
 $string['plugindescription'] = 'This authentication plugin displays a list of the configured identity providers on the login page. Selecting an identity provider allows users to login with their credentials from an OAuth 2 provider.';
 $string['pluginname'] = 'OAuth 2';
+$string['alreadylinked'] = 'This external account is already linked to an account on this site';
diff --git a/auth/oauth2/linkedlogins.php b/auth/oauth2/linkedlogins.php
index 12285306a5a..fe18f8fb847 100644
--- a/auth/oauth2/linkedlogins.php
+++ b/auth/oauth2/linkedlogins.php
@@ -58,8 +58,12 @@ if ($action == 'new') {
     $userinfo = $client->get_userinfo();
 
     if (!empty($userinfo)) {
-        \auth_oauth2\api::link_login($userinfo, $issuer);
-        redirect($PAGE->url, get_string('changessaved'), null, \core\output\notification::NOTIFY_SUCCESS);
+        try {
+            \auth_oauth2\api::link_login($userinfo, $issuer);
+            redirect($PAGE->url, get_string('changessaved'), null, \core\output\notification::NOTIFY_SUCCESS);
+        } catch (Exception $e) {
+            redirect($PAGE->url, $e->getMessage(), null, \core\output\notification::NOTIFY_ERROR);
+        }
     } else {
         redirect($PAGE->url, get_string('notloggedin', 'auth_oauth2'), null, \core\output\notification::NOTIFY_ERROR);
     }
diff --git a/auth/oauth2/version.php b/auth/oauth2/version.php
index 8fc34eb410d..275893493ad 100644
--- a/auth/oauth2/version.php
+++ b/auth/oauth2/version.php
@@ -24,6 +24,6 @@
 
 defined('MOODLE_INTERNAL') || die();
 
-$plugin->version   = 2017051500;        // The current plugin version (Date: YYYYMMDDXX).
+$plugin->version   = 2017051501;        // The current plugin version (Date: YYYYMMDDXX).
 $plugin->requires  = 2017050500;        // Requires this Moodle version.
 $plugin->component = 'auth_oauth2';       // Full name of the plugin (used for diagnostics).