How to create a "waiting" queue when there are more than 2 visitors on the website

Hello, I use a form on my website. This form contains content that can only be selected once.

I would like to create some sort of "queue" when there is more than one visitor on the page.

So for example 2 visitors: visitor 1 -> fill in the form. visitor 2 -> gets popup with "please wait" visitor 1 -> sends the form. system processes data from form. visitor 2 -> make popup invisible and show form.

Or another option would be to check if someone else has checked the checkbox yet and give a message if someone has checked this yet.