解决方案
我在while循环外创建了数组，并检查值是否在数组中。
<?php
$args = array(\'post_type\' => \'posttype\');
$emptyvalue = "";
$optionname = "optionname";
$the_query = new WP_Query($args);
$output ="<select name=\'".$optionname."\'> <option value=\'".$emptyvalue."\'>Location</option>\'";
$arr = array();
while ( $the_query->have_posts() ) : $the_query->next_post();
$id= $the_query->post->ID;
$location = get_post_meta($id, \'institution_location\', true);
if(!in_array($location, $arr)){
array_push($arr, $location);
$get_location = $_GET[\'institution-location\'];
$selected = ($get_location == $location) ? \'selected=selected\' : \'\';
$output .="<option $selected name=\'".$location."\' value=\'".$location."\'>".$location." </option>";
}
endwhile;
$output .="</select>";
echo $output;
?>
