// Check if the tag exists already
$query = "SELECT id FROM #__k2_tags WHERE name=".$db->Quote($itemTag);
$db->setQuery($query);
$id = $db->loadResult();
if ($id) {
    $query = "INSERT INTO #__k2_tags_xref (`id`, `tagID`, `itemID`) VALUES (NULL, {$id}, {$K2Item->id})";
} else {
    $K2Tag = JTable::getInstance('K2Tag', 'Table');
    $K2Tag->name = $itemTag;
    $K2Tag->published = 1;
    $K2Tag->store();
    $tags[] = $K2Tag;
    $query = "INSERT INTO #__k2_tags_xref (`id`, `tagID`, `itemID`) VALUES (NULL, {$K2Tag->id}, {$K2Item->id})";
}
$db->setQuery($query);
$db->query();