JSFiddle - React, Tailwind, and code Playground

HTML

<select id="sel">
    <option value="foo">foo</option>
</select>
<button data-add="foo">add foo</button>
<button data-add="bar">add bar</button>

JavaScript

$('button').on('click', function() {
debugger;
    var new_val = $(this).data('add');
    
    if ($('option[value='+new_val+']', '#sel').length) {
        alert('nope, exists!')
    } else {
            
        $('<option value="'+ new_val +'">'+new_val+'</option>').appendTo( $('#sel') );
    }
});