How to Remove duplicate dropdown option elements with same value

Using .siblings() (to target sibling option elements), and Attribute Equals Selector [attr=""]

$(".select option").val(function(idx, val) {
  $(this).siblings('[value="'+ val +'"]').remove();
});

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>

<select class="select">
  <option value="">All</option>
  <option value="com">.com 1</option>
  <option value="net">.net 1</option>
  <option value="com">.com 2</option> <!-- will be removed since value is duplicate -->
  <option value="net">.net 2</option> <!-- will be removed since value is duplicate -->
</select>

(works also for multiple .select on the same page)
^{I added a class .select to the <select> element to be more selector-specific}

How it works:
while options are accessed one by one (by .val()) - lookup for .sibling() options that have the same "[value='"+ this.value +"']" and .remove() them.

The correct solution will be is to not to allow the server to have duplicate values....

try

var map = {};
$('select option').each(function () {
    if (map[this.value]) {
        $(this).remove()
    }
    map[this.value] = true;
})

Demo: Fiddle

use this :

$(document).ready(function () {
    var usedNames = {};
    $("select > option").each(function () {
        if (usedNames[this.value]) {
            $(this).remove();
        } else {
            usedNames[this.value] = this.text;
        }
    });
});

demo here : http://jsfiddle.net/aelor/aspMT/