[jQuery] Filter doubles from selection

[Korijn]

The problem I have is (I think) quite simple but I find myself unable
to find a solution to it.
In the system I made you can select options from a select box and then
click a button to add them to a different select box with your
selected options. There's also a button to remove items from your
selection. This works fine and you can see it below.

What I need is something added to the first selector that will make
sure of the following:
If an option has already been added to #selection it should not be
added again! But how can I accomplish that?
Thanks in advance!

Here's the (stripped) situation:

<script type="text/javascript">
//<![CDATA[
$(document).ready(function() {
        $("#right").click(function() {
                $("#options option:selected").clone().appendTo("#selection");
                return(false);
        });
        $("#left").click(function() {
                $("#selection option:selected").remove();
                return(false);
        });
});
//]]>
</script>

<select id="options" multiple="multiple">
<option value="1">test 1</option>
<option value="2">test 2</option>
<option value="3">test 3</option>
</select>

<button id="right">&gt;&gt;</button>
<button id="left">&lt;&lt;</button>

<select id="selection" multiple="multiple">
</select>