check boxes and URL rewriting

Hi Guys, Hope someone can help me with my code. i am using rewritten URL's and have this piece of code on a form. When the page first loads mypage.htm, the type_24 checkbox get's checked by default. If they check the type_12 box, i want the type_24 box to uncheck.

My problem is, if i check the type_12 box, the page refreshes and both the type_12 and type_24 boxes are checked.. which is not what i want. I think it's because i'm reloading my rewritten URL in my action because it works fine if i just have php file as the action.

Any ideas how i can fix my code so that it only has the type_12 checked when i check the type_24 box?

<form name="frmrefresh" id="frmrefresh" method="post" action="mypage.htm">

<input type="checkbox" name="type_12" id="type_12" <?php if(isset($_POST['type_12']) && $_POST['type_12']=="12"){?> checked="checked"<?php }?> value="12" onClick="uncheck24(this);" /> <label>12</label>

<input type="checkbox" name="type_24" id="type_24" <?php if(isset($_GET['id']) && $_GET['id']!=''){?>checked="checked"<?php }?><?php if(isset($_POST['month_24']) && $_POST['type_24']=="24"){?> checked="checked"<?php }?> value="24" onClick="uncheck12(this);"/> <label>24 Months</label>

<input type="hidden" name="id" value="<?php echo $_REQUEST['id'];?>" />
</form>

In the header i have the functions:

function uncheck12(obj)
{
if (obj.checked == true)
{
document.getElementById("type_12").checked = false;
document.frmrefresh.submit();
}
}

function uncheck24(obj)
{
if (obj.checked == true)
{
document.getElementById("type_24").checked = false;
document.frmrefresh.submit();
}
}

Lots of code to do what RADIO buttons do. Just have mypage.htm check their status and refresh the page with the correct one set.http://www.echoecho.com/htmlforms10.htm