Multiple textbox value change (date)

[xox]

For example, I've got 10 input boxes (textbox) and 5 of them must contain date that user enters using calender script, the problem is, all of the boxes that requires date as input have the same id and now only first textbox is populated (logic I know), how can I populate the other ones?

 

Code for showing calendar images next to textboxes which IDs are "a":

if ($row['fieldtype'] == 'a')
				{
					echo '<input type="text" id="a" name="textbox[]" value=""></td><td>';
					echo '<input type="hidden" id="a" name="textbox[]" value=""></td><td>';
					echo '<a href="javascript:viewcalendar()"><img src="calendar.png"></a></td><td>';
				}

 

And the code in scripts.js that populates fields

function insertdate(d) {
  window.close();
  window.opener.document.getElementById('a').value = d;
}

[xox]

I've forgot to write that fields are generated depending on number of values in DB, it could show low as 1 field and up to 40 fields. And there can be none date required field or unlimited.

[xox]

No one?