hook_form_alter function does not call the validate function?
as your validating just one field - at least, it seems like it; consider using:

function uniquefield_form_alter(&amp;$form, &amp;$form_state, $form_id) {
  if ($form_id == 'target_achieved_node_form' || $form_id ==  'target_node_form') {
    $form['your_desired_field']['#element_validate']  = array('uniquefield_validate');
  }
}


on the fileds in your alter hook