JSFiddle - React, Tailwind, and code Playground

HTML

<form name="input" method="post" action="#">Display Tag:
    <input type="text" name="taginput">
    <input type="submit" value="Go">
    <select name="tags" multiple="multiple">
        <option value="C#">C#</option>
        <option value="Java">Java</option>
        <option value="Javascript">Javascript</option>
        <option value="PHP">PHP</option>
        <option value="Android">Android</option>
        <option value="jQuery">jQuery</option>
        <option value="C++">C++</option>
        <option value="Python">Python</option>
        <option value="HTML">HTML</option>
        <option value="MySQL">MySQL</option>
    </select>
</form>

JavaScript

var sel = document.getElementsByName('tags')[0];
var choosen = [];
sel.onclick = function () {
    var is_there = !!~choosen.indexOf(this.value);
    if(is_there){return false;};
    choosen.push(this.value);
    document.getElementsByName('taginput')[0].value += this.value + ' ';
}