JSFiddle - React, Tailwind, and code Playground

HTML

<input type="text" name="class1" placeholder="Class Name1?" required="">
<input type="text" name="class2" placeholder="Class Name2?" required="">
<input type="text" name="class3" placeholder="Class Name3?" required="">
<input type="text" name="class4" placeholder="Class Name4?" required="">
<input type="text" name="class5" placeholder="Class Name5?" required="">

JavaScript

$(document).ready(function () {
    $('input[type=text]').bind('change', function () {
        var arr = []
        $siblings = $(this).siblings();
        $.each($siblings, function (i, key) {
           arr.push($(key).val()); 
        });
        if ($.inArray($(this).val(), arr) !== -1)
        {
            alert("duplicate has been found");
        }
    });    
});