getTvUnique.php

<?php
$output = array();

$q = $modx->newQuery('modTemplateVarResource');
$q->select('DISTINCT(`modTemplateVarResource`.`value`)');
$q->innerJoin('modTemplateVar', 'tv', "tv.id = modTemplateVarResource.tmplvarid");
$q->innerJoin('modResource', 'res', 'res.id=modTemplateVarResource.contentid');
$q->where(array(
		'tv.name' => 'category', 
		'res.context_key' => $modx->resource->context_key,
		'res.parent' => 17
	)
);
$res = '';
if ($q->prepare() && $q->stmt->execute()) {
	while ($row = $q->stmt->fetch(PDO::FETCH_ASSOC)) {
		$output[] = $row['value'];
		$res .= '<option value="'.$row['value'].'">'.$row['value'].'</option>';
	}
}

return $res;