diff --git a/CHANGELOG.md b/CHANGELOG.md
index b9eabc7e..a8a8edc2 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -5,6 +5,7 @@
 - It’s now possible to set the “Config Options” setting to a JS or JSON file within `config/ckeditor/`. ([#540](https://github.com/craftcms/ckeditor/pull/540))
 - It’s now possible to set the “Custom Styles” setting to a CSS file within `config/ckeditor/`. ([#540](https://github.com/craftcms/ckeditor/pull/540))
 - The v5 migration now automatically creates JS/JSON/CSS files within `config/ckeditor/` for any Config Options/Custom Styles from CKEditor Configs that were used by two or more fields. ([#540](https://github.com/craftcms/ckeditor/pull/540))
+- Fixed an error that could occur when registering a custom CKEditor plugin. ([#531](https://github.com/craftcms/ckeditor/issues/531))
 
 ## 5.2.1 - 2026-03-18
 
diff --git a/src/helpers/CkeditorConfig.php b/src/helpers/CkeditorConfig.php
index 55ee8ac1..6e7c9335 100644
--- a/src/helpers/CkeditorConfig.php
+++ b/src/helpers/CkeditorConfig.php
@@ -215,7 +215,12 @@ public static function registerPackage(string $name, array $config): void
         $plugins = $config['plugins'] ?? [];
         $toolbarItems = $config['toolbarItems'] ?? [];
 
-        self::$pluginsByPackage[$name] = $plugins;
+        if (!isset(self::$pluginsByPackage[$name])) {
+            self::$pluginsByPackage[$name] = $plugins;
+        } else {
+            self::$pluginsByPackage[$name] = array_unique(array_merge(self::$pluginsByPackage[$name], $plugins));
+        }
+
         self::$toolbarItems[] = $toolbarItems;
         self::$pluginButtonMap[] = [
             'plugins' => $plugins,