$('.pool').on('mouseleave', function (e) {                
            var el = $(this).find('.pool-row').find('input[type=submit]');

            if (el.length)
                alert('bla bla bla');

            e.stopImmediatePropagation();
            e.stopPropagation();
            return;
        });

The alert is triggering twice on mouseleave even with stopPropagation or preventDefault and "return;".

>Solution :

I have made some quick modifications on your script, and it seems that it does not trigger the alert twice!

$('.pool').on('mouseleave', function (e) {
  e.stopPropagation()
  var el = $(this).find('input[type="submit"]')

  if (el.length)
      alert('bla bla bla')
})

<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
<div class="pool">
  <p>This is a pool</p>
  
  <input type="submit">
</div>