diff --git a/src/script.php b/src/script.php
index c48027e..35f6641 100644
--- a/src/script.php
+++ b/src/script.php
@@ -94,6 +94,7 @@ class Tpl_MokoonyxInstallerScript implements InstallerScriptInterface
             $this->clearFaviconStamp();
             $this->cleanMediaFolder();
             $this->removeDeletedFiles();
+            $this->removeDuplicateExtensions();
             $this->lockExtension();
         }
 
@@ -484,6 +485,96 @@ class Tpl_MokoonyxInstallerScript implements InstallerScriptInterface
         }
     }
 
+    /**
+     * Remove duplicate MokoOnyx extension entries from #__extensions.
+     *
+     * Re-installs or migrations can leave ghost rows.  We keep the one
+     * that is locked (the active template) and delete any extras.
+     * Also removes stale MokoCassiopeia entries if present.
+     */
+    private function removeDuplicateExtensions(): void
+    {
+        $db = Factory::getDbo();
+
+        // Find all MokoOnyx template entries
+        $rows = $db->setQuery(
+            $db->getQuery(true)
+                ->select(['extension_id', 'locked'])
+                ->from('#__extensions')
+                ->where($db->quoteName('element') . ' = ' . $db->quote(self::NEW_NAME))
+                ->where($db->quoteName('type') . ' = ' . $db->quote('template'))
+                ->order('locked DESC, extension_id ASC')
+        )->loadObjectList();
+
+        if (count($rows) > 1) {
+            $keep = (int) $rows[0]->extension_id;
+            $removed = 0;
+
+            for ($i = 1; $i < count($rows); $i++) {
+                $staleId = (int) $rows[$i]->extension_id;
+
+                $db->setQuery(
+                    $db->getQuery(true)
+                        ->delete('#__update_sites_extensions')
+                        ->where('extension_id = ' . $staleId)
+                )->execute();
+
+                $db->setQuery(
+                    $db->getQuery(true)
+                        ->delete('#__extensions')
+                        ->where('extension_id = ' . $staleId)
+                )->execute();
+
+                $removed++;
+            }
+
+            if ($removed > 0) {
+                $this->logMessage("Removed {$removed} duplicate MokoOnyx extension(s). Kept ID {$keep}.");
+            }
+        }
+
+        // Remove stale MokoCassiopeia if not set as default
+        $oldExt = (int) $db->setQuery(
+            $db->getQuery(true)
+                ->select('extension_id')
+                ->from('#__extensions')
+                ->where($db->quoteName('element') . ' = ' . $db->quote(self::OLD_NAME))
+                ->where($db->quoteName('type') . ' = ' . $db->quote('template'))
+        )->loadResult();
+
+        if ($oldExt) {
+            $isDefault = (int) $db->setQuery(
+                $db->getQuery(true)
+                    ->select('COUNT(*)')
+                    ->from('#__template_styles')
+                    ->where($db->quoteName('template') . ' = ' . $db->quote(self::OLD_NAME))
+                    ->where($db->quoteName('home') . ' = 1')
+            )->loadResult();
+
+            if ($isDefault === 0) {
+                $db->setQuery(
+                    $db->getQuery(true)
+                        ->delete('#__update_sites_extensions')
+                        ->where('extension_id = ' . $oldExt)
+                )->execute();
+
+                $db->setQuery(
+                    $db->getQuery(true)
+                        ->delete('#__extensions')
+                        ->where('extension_id = ' . $oldExt)
+                )->execute();
+
+                $db->setQuery(
+                    $db->getQuery(true)
+                        ->delete('#__template_styles')
+                        ->where($db->quoteName('template') . ' = ' . $db->quote(self::OLD_NAME))
+                )->execute();
+
+                $this->logMessage('Removed stale MokoCassiopeia extension and styles.');
+            }
+        }
+    }
+
     /**
      * Remove files and directories that were shipped in previous versions
      * but have since been deleted from the package.