DOCS-401 minor edit

Author: Bob Grabar

source/administration/replica-sets.txt

@@ -935,9 +935,8 @@ Duplicate Key Error on local.slaves
 The *duplicate key on local.slaves* error, shown below, occurs when a
 :term:`secondary` or :term:`slave` changes its hostname and the
 :term:`primary` or :term:`master` tries to update its ``local.slaves``
-collection with the new name but runs into a duplicate `_id` field. The
-duplicate `_id` field exists in a document containing the previous
-hostname.
+collection with the new name. The update fails because it contains the
+same `_id` value as the document containing the previous hostname.
 
 .. example::