Hello - fairly new to javascript - am hoping someone can help?

I am using this script:

Code:
<script language="javascript">
function printdiv(printpage)
{
var headstr = "<html><head><title></title></head><body>";
var footstr = "</body>";
var newstr = document.all.item(printpage).innerHTML;
var oldstr = document.body.innerHTML;
document.body.innerHTML = headstr+newstr+footstr;
window.print();
document.body.innerHTML = oldstr;
return false;
}
</script>
And then this HTML:
HTML Code:
<div id="div_print">
<img src="service/the-original-29.95-oil-change.jpg" width="900" height="483"> </div>
<div class="print-button">
<span style="font-family:Arial, Helvetica, sans-serif; font-size:14px; color:#F00; font-weight:bold;">Please print this coupon and present to dealer at time of service</span><br /><br />
<input name="b_print" type="button" class="ipt"   onClick="printdiv('div_print');" value=" Print ">
</div><!--print-button-->
This works exactly as I hoped. The problem arrises when I try to add a second instance, for example:

HTML Code:
<div id="div_print">
<img src="service/the-original-29.95-oil-change.jpg" width="900" height="483"> </div>
<div class="print-button">
<span style="font-family:Arial, Helvetica, sans-serif; font-size:14px; color:#F00; font-weight:bold;">Please print this coupon and present to dealer at time of service</span><br /><br />
<input name="b_print" type="button" class="ipt"   onClick="printdiv('div_print');" value=" Print ">
</div><!--print-button-->

<br /><br />

<div id="div_print">
<img src="service/tire-rotation.jpg" width="900" height="483"> </div>
<div class="print-button">
<span style="font-family:Arial, Helvetica, sans-serif; font-size:14px; color:#F00; font-weight:bold;">Please print this coupon and present to dealer at time of service</span><br /><br />
<input name="b_print" type="button" class="ipt"   onClick="printdiv('div_print');" value=" Print ">
</div><!--print-button-->
No it does not work at all - both print buttons give the error: "Undefined". I was hoping to just use the same div container and just change the contents within that div.

I appreciate any help /comments that can help me out.

Thanks,

Abe