Popup alert even when a site deletes the alert function | bypass

A website fully filter the alert function from his website, and replace it with an empty string, but I want to bypass it and still popup an alert, I am trying to solve an XSS challenge, and I figure out that the site identify the double "l" char, and fully removes the string.

http://alertmywebsite.com/2.php?xss=<script>allert(1)</script>

the output is <script>allert(1)</script>, and when I remove the second "l", the output is <script></script>, Only the "l" is still showing the alert function, any other double char is fully removes the string How can I bypass it ?