JSFiddle - React, Tailwind, and code Playground

HTML

<form>
  USER 1
  <input name="user1" value="bla1">
  <input name="pass1" value="ple1">
  <input name="email1" value="[email protected]">

  USER 2
  <input name="user2" value="bla2">
  <input name="pass2" value="ple1">
  <input name="email2" value="[email protected]">

  USER 3
  <input name="user3" value="bla1">
  <input name="pass3" value="pla3">
  <input name="email3" value="[email protected]">

  <button type="submit">
</form>

CSS

input.error {background:red}

JavaScript

$('form').submit(function(event) {
		var ok = true,
			$form = $(this),
			$inputs = $(this).find('input')
		$inputs.removeClass('error')
		$inputs.each(function() {
			var $src = $(this)
			
			//Get the name of the element, and strip off the number
			var name = $(this).attr('name').replace(/[0-9]/g,'')
			
			//Filter the inputs down to only the ones whose name starts the same
			$inputs.not(this).filter('[name^="' + name + '"]').each(function() {
               if($src.val() == $(this).val()) {
					ok = false
					$(this).addClass('error')
					$src.addClass('error')
				}
			})
		})
		if(!ok) event.preventDefault()
	})