diff --git a/includes/class-webmention.php b/includes/class-webmention.php
index f988547..167d7cb 100644
--- a/includes/class-webmention.php
+++ b/includes/class-webmention.php
@@ -34,6 +34,13 @@ class Webmention {
 	 */
 	private $default_post_types = array( 'post', 'page' );
 
+	/**
+	 * Whether the class has been initialized.
+	 *
+	 * @var boolean
+	 */
+	private $initialized = false;
+
 	/**
 	 * Get the instance of the class.
 	 *
@@ -58,6 +65,10 @@ private function __construct() {
 	 * Initialize the plugin.
 	 */
 	public function init() {
+		if ( $this->initialized ) {
+			return;
+		}
+
 		$this->register_constants();
 
 		$this->register_hooks();
@@ -67,6 +78,8 @@ public function init() {
 
 		// Load language files.
 		load_plugin_textdomain( self::TEXT_DOMAIN, false, dirname( plugin_basename( __FILE__ ) ) . '/languages' );
+
+		$this->initialized = true;
 	}
 
 	/**
diff --git a/tests/test-webmention.php b/tests/test-webmention.php
index 378a06a..afdc05a 100644
--- a/tests/test-webmention.php
+++ b/tests/test-webmention.php
@@ -1,4 +1,21 @@
 <?php
 class Webmention_Test extends WP_UnitTestCase {
+	public function test_register_constants() {
+		\Webmention\Webmention::get_instance()->init();
+
+		$this->assertEquals( WEBMENTION_ALWAYS_SHOW_HEADERS, 0 );
+		$this->assertEquals( WEBMENTION_COMMENT_APPROVE, 0 );
+		$this->assertEquals( WEBMENTION_COMMENT_TYPE, 'webmention' );
+		$this->assertEquals( WEBMENTION_GRAVATAR_CACHE_TIME, WEEK_IN_SECONDS );
+	}
+
+	public function test_register_hooks() {
+		\Webmention\Webmention::get_instance()->init();
+
+		// test if some hooks are registered
+		$this->assertEquals( has_action( 'init', array( \Webmention\Comment::class, 'init' ) ), 10 );
+		$this->assertEquals( has_action( 'admin_menu', array( \Webmention\Admin::class, 'admin_menu' ) ), 10 );
+		$this->assertEquals( has_action( 'admin_init', array( \Webmention\Admin::class, 'admin_init' ) ), 10 );
+	}
 
 }