[C++] Add documentation warning against the use of duplicate key names (#101)

* add note about duplicate keys

Author: rachel-mack

source/data-formats/working-with-bson.txt

@@ -28,6 +28,11 @@ and supports additional types, including dates, different-sized integers, ``Obje
 values, and binary data. For a complete list of supported types, see the :manual:`BSON Types </reference/bson-types/>`
 page in the {+mdb-server+} manual. 
 
+.. note::
+   
+   Avoid adding duplicate keys to BSON documents. Although the `BSON specification <https://bsonspec.org/>`__ 
+   allows duplicate keys, they can cause unexpected behavior in {+mdb-server+}.
+
 BSON Values and Views
 ~~~~~~~~~~~~~~~~~~~~~