Fix

Author: mcmorisi

source/write/bulk-write.txt

@@ -67,6 +67,12 @@ The following example creates an instance of ``InsertOne``:
 
 To insert multiple documents, create an instance of ``InsertOne`` for each document.
 
+.. note::
+
+   Duplicate ``_id`` values violate unique index constraints, which causes the
+   driver to return a ``DuplicateKeyError``. To avoid this error, ensure that
+   each document you insert has a unique ``_id`` value.
+
 Update Operations
 ~~~~~~~~~~~~~~~~~