JSFiddle - React, Tailwind, and code Playground

HTML

<script src="https://code.jquery.com/jquery-2.2.1.min.js"></script>
<form action="" method="post">
  <p>
    <label for="email0">Email 0 : </label>
    <input type="mail" class="contactpersonen-email" id="email0" value="foo@bar.com" />
  </p>
  <p>
    <label for="email1">Email 1 : </label>
    <input type="mail" class="contactpersonen-email" id="email1" value="bar@foo.com" />
  </p>
  <p>
    <label for="email2">Email 2 : </label>
    <input type="mail" class="contactpersonen-email" id="email2" value="foo@foo.com" />
  </p>
  <input type="submit" />
</form>

JavaScript

Array.prototype.areUnique = function() {
	var l = this.length;
  for(var i=0; i<l; i++) {
  	for(var j=i+1; j<l; j++) {
    	if (this[i] === this[j]) return false;
    }
	}
	return true;
};

$(document).ready(function(){
	$('form').submit(function(){
  	var mails = $('.contactpersonen-email').map(function(){
        return $(this).val();
    }).get(); 
    
    if (!mails.areUnique()) {
    	alert('values are not unique');
    } else {
    	alert('values are unique');
    }
    return false;
  });
});