JSFiddle - React, Tailwind, and code Playground

HTML

<script src="https://ajax.aspnetcdn.com/ajax/jquery.validate/1.13.1/jquery.validate.js"></script>
	<form id="signupForm" method="get" action="">
		<fieldset>			
			<p>
				<label for="username">Username</label>
				<input name="username[]" type="text">
			</p>
		    <p>
				<label for="username">Username</label>
				<input name="username[]" type="text">
			</p>

			<p>
				<input class="submit" type="submit" value="Submit">
			</p>
		</fieldset>
	</form>

JavaScript

$("#signupForm").validate({
    rules: {
        'username[]': "required"
    },
    messages: {
        'username[]': {
		    required: "Please enter a username",
		}
	}
});