That's because your hidden fields have duplicate IDs, so jQuery only returns the first in the set. Give them classes instead, like .uid and grab them via:
var uids = $(".uid").map(function() {
return this.value;
}).get();
Demo: http://jsfiddle.net/karim79/FtcnJ/
EDIT: say your output looks like the following (notice, IDs have changed to classes)
<fieldset><legend>John Smith</legend>
<img src='foo.jpg'/><br>
<a href="#" class="aaf">add as friend</a>
<input name="uid" type="hidden" value='<?php echo $row->uid;?>' class="uid">
</fieldset>
You can target the 'uid' relative to the clicked anchor like this:
$("a.aaf").click(function() {
alert($(this).next('.uid').val());
});
Important: do not have any duplicate IDs. They will cause problems. They are invalid, bad and you should not do it.