JSFiddle - React, Tailwind, and code Playground

HTML

<select id="items">
    <option>One</option>
    <option>Two</option>
    <option>Three</option>
</select>
<input type="button" id="add" value="Add" />
<select id="result" multiple="multiple" size="5">
</select>

JavaScript

$('#add').click(function(){
    var item = $('#items').val();
    var $result = $('#result>option[value="' + item + '"]');
    if($result.length == 0){
        $('#result').append('<option value="' + item + '">' + item + '</option>');
    }
    else{
       alert('The item "' + item + '" has already been added');   
    }
});