I think your situation is a good use case for built-in options to enable/disable case-sensitive comparison in field rules, however, at the moment I don't think there is a simple out-of-the-box solution.

Are you using the classic designer or the modern designer? If you're using the classic designer, you could use JavaScript to create custom validation rules.

The biggest downside to relying on normal field rules is that the warning alone would not prevent the user from ignoring it and submitting anyway.

If you are able to use custom validation rules in a classic form, you can leverage the built-in validation to both show the error message and prevent submission.

In the following example (classic designer), each field with the uniqueemail class will have the validator assigned to require uniqueness, which makes it pretty easy to scale because you only need to add the class to each field.

$(document).ready(function (){
  // custom validator for email addresses
  window.Parsley.addValidator('uniqueemail', {
    validateString: function(value, requirement, field) {
      // convert to lower case and set default result
      value = value.toLowerCase();
      var isValid = true;
      
      $('.uniqueemail input').each(function (){
        // check values for all other fields except self
        if(!$(this).is(field.$element) && $(this).val().toLowerCase() == value){
          isValid = false;
        }
      });
      
      return isValid;
    },
    messages: {
      en: 'Email address already used.',
    }
  });
  
  // assign validator to email field
  $('.uniqueemail input').attr('data-parsley-uniqueemail','');
});

There's a lot of different ways you could handle the actual comparison, but this is a quick example of how the custom validator approach could work.

A little more code would be needed if you're dealing with tables/collections and some of the email fields may be added after the form has already loaded.