Feeds XPath Parser - Update existing content
it doesn't recognized the nid field as unique, 

What is  the field associated with the nid value? 

If you have a custom field associated with the nid :

You should install https://www.drupal.org/project/field_validation

and add the validation of the field at 
http://yourwebsite/admin/structure/field_validation

and set the validator with the value "Unique values".

Sometimes a space in the value of the feed can cause this in this case you can use tamper for trim spaces :
https://www.drupal.org/project/feeds_tamper