Fix: Handle duplicate mark instances correctly in DOMSerializer

[o1y]

Fixes a bug where duplicate mark types (e.g., multiple textStyle marks on the same node) were incorrectly handled, causing marks to stay open.

Example:

$document = [
      'type' => 'doc',
      'content' => [
          [
              'type' => 'paragraph',
              'content' => [
                  [
                      'type' => 'text',
                      'text' => 'Example',
                      'marks' => [
                          [
                              'type' => 'bold',
                          ],
                          [
                              'type' => 'textStyle',
                          ],
                          [
                              'type' => 'textStyle', // duplicate
                          ],
                      ],
                  ],
                  [
                      'type' => 'text',
                      'text' => ' Text',
                  ],
              ],
          ],
      ],
  ];

Before:

<p><strong><span><span>Example</span></span></strong><span> Text</p>

After:

<p><strong><span><span>Example</span></span></strong> Text</p>