Update imported feeds to only get one taxonomy term per node
If data needs to be modified before it is saved use this module : Feeds tamper

For advanced modification use hook_nodeapi in drupal 6 with the insert parameter , with hook_nodeapi load the old node like this :

$old_node = node_load(node-&gt;nid);
$node-&gt;taxonomy = array_diff($old_node-&gt;taxonomy,$node-&gt;taxonomy)


So now you are sure that the $node contain only the new value , Hope this  will help