DOM Event fires twice?

I am using the DOM event wpcf7mailsent to track some FB events. Perhaps there was a change in CF7 5.8+ which leads to firing this event twice.

For example, I have this simple snippet:

<script>
document.addEventListener( 'wpcf7mailsent', function( event ) { 
alert('Success!'); 
fbq('track', 'Lead'); 
}, false ); 
</script>

Because the Facebook Pixel Helper complained about multiple events fired, I added the alert, which got executed two times.

Any idea what can cause this? I don’t think I have changed anything what could cause this. Could this be a bug in CF7?

I tried to reproduce this issue, but I had no luck in a local environment. Any idea how to further debug this type of problem? Does anyone have the same issue?

All the best
Torsten