JSFiddle - React, Tailwind, and code Playground

HTML

<form id="form">
    <input class="email" id="email1"><br>
    <input class="email" id="email2"><br>
    <input class="email" id="email3"><br>
    <input class="email" id="email4"><br>
    <input class="email" id="email5">
</form>
<button id="cmd">check</button>

JavaScript

$('#cmd').click(function(){
    var duplicate=false;
    
    $('input[id^=email]').each(function(){
        var $this = $(this);
        if ($this.val()===''){ return;}
        $('input[id^=email]').not($this).each(function(){
            if ( $(this).val()==$this.val()) {duplicate=true;}
        });
    });
    
    if(duplicate) alert('email must be valid');
});