JSFiddle - React, Tailwind, and code Playground

by ChaseWest

HTML

<select id="test1" multiple>
    <option>1</option>
    <option>2</option>
    <option>3</option>
    <option>4</option>
    <option>5</option>
</select> 

<select id="test2" multiple>
    <option>1</option>
    <option>2</option>
    <option>3</option>
    <option>4</option>
    <option>5</option>
</select> 

<button>run</button>

JavaScript

$("button").on("click", function(){
    
    var holder = [];
    
    $("select option:selected").each(function(i){
        if(holder.indexOf(this.value) === -1){
            holder.push(this.value);
        }
    });
    
    $("select").val(holder);
});