How to skip (don't update) some fields from existing node during import if this field already has a value?
Had the same problem. I finally used hook_feeds_presave(), and unset the field I wanted to skip. Here's the code:

function mymodule_feeds_presave (FeedsSource $source, $entity, $item, $entity_id) {
  if ($source-&gt;importer-&gt;id == 'myimporter'){
    // I check if my imported value is equal to a special marker i set
    if ($entity-&gt;my_field['und'][0]['value'] == 'MARKER') {
      unset($entity-&gt;field_i_wanted_to_skip);
    }
  }
}


This automatically resets THAT FIELD ONLY. Hope this helps someone.