JSFiddle - React, Tailwind, and code Playground

HTML

<form method="POST">
    <select name="proizvod[]">
        <option value="1">nesto</option>
        <option value="2">nesto1</option>
        <option value="3">nesto2</option>
    </select>
    <select name="proizvod[]">
        <option value="1">nesto</option>
        <option value="2">nesto1</option>
        <option value="3">nesto2</option>
    </select>
    <button type="submit">Submit</button>
</form>

JavaScript

jQuery(document).ready(function () {
    jQuery('button[type="submit"]').click(function () {

        jQuery('option:selected').each(function () {
            if (jQuery('option:selected[value="' + jQuery(this).attr('value') + '"]').length > 1) {
                alert('Error!');
                return false;
            }

        });

    });
});