diff --git a/source/data-modeling/indexes.txt b/source/data-modeling/indexes.txt
index dcf3989..ba7dac6 100644
--- a/source/data-modeling/indexes.txt
+++ b/source/data-modeling/indexes.txt
@@ -5,8 +5,6 @@
 Optimize Queries With Indexes
 =============================
 
-.. default-domain:: mongodb
-
 .. facet::
    :name: genre
    :values: reference
@@ -59,6 +57,16 @@ index and its direction. ``1`` represents an ascending index, and ``-1`` represe
 descending index. The second hash object contains index options. To learn more
 about index options, see the :ref:`mongoid-indexes-api-documentation` section.
 
+.. note:: Duplicate Indexes
+   
+   {+odm+} might incorrectly label some index declarations as duplicates
+   and silently prevent index creation. To change this default behavior and push
+   index verification to the server, you can set the
+   ``Mongoid.allow_duplicate_index_declarations`` configuration option to ``true``.
+   Then, if the server detects a duplicate index, it raises an error.
+   To learn more, see the :ref:`mongoid-config-options-all` section of the
+   Application Configuration guide.
+
 Aliases and Declaring Indexes
 +++++++++++++++++++++++++++++
 
@@ -217,4 +225,4 @@ To learn more about index options, see the `Mongoid::Indexable::Validators::Opti
 
 To learn more about using Atlas Search indexes in {+odm+}, see the 
 `Mongoid::SearchIndexable::ClassMethods <{+api+}/Mongoid/SearchIndexable/ClassMethods.html>`__
-documentation.
\ No newline at end of file
+documentation.
diff --git a/source/includes/configuration/sample-config-options.yml b/source/includes/configuration/sample-config-options.yml
index 9667583..a1ed6ab 100644
--- a/source/includes/configuration/sample-config-options.yml
+++ b/source/includes/configuration/sample-config-options.yml
@@ -123,6 +123,14 @@ development:
     # field values. Only has effect when BSON 5+ is present.
     allow_bson5_decimal128: false
 
+    # Allows duplicate indexes to be declared and sent
+    # to the server. The server then validates the indexes and raises an
+    # exception if duplicate indexes are detected. When false, indexes
+    # are validated on the client, which can lead to some indexes
+    # getting silently ignored even if they are not duplicates.
+    # (default: false)
+    allow_duplicate_index_declarations: false
+
     # Sets app name that is printed to the MongoDB logs upon establishing
     # a connection. Value is used as the database name if the database
     # name is not provided. (default: nil)
diff --git a/source/whats-new.txt b/source/whats-new.txt
index fda72e8..86930e8 100644
--- a/source/whats-new.txt
+++ b/source/whats-new.txt
@@ -12,11 +12,26 @@ What's New
 
 Learn what's new in:
 
+* :ref:`Version 9.1 <mongoid-version-9.1>`
 * :ref:`Version 9.0 <mongoid-version-9.0>`
 
 To view a list of releases and detailed release notes, see {+odm+}
 :github:`Releases <mongodb/mongoid/releases>` on GitHub.
 
+.. _mongoid-version-9.1:
+
+What's New in 9.1
+-----------------
+
+The 9.1 release includes the following new features, improvements, and
+fixes:
+
+- Adds the ``Mongoid.allow_duplicate_index_declarations``
+  configuration option that enables you to send index verification to
+  the server. If a duplicate index is declared, the server raises an
+  error. To learn more, see the :ref:`mongoid-config-options-all`
+  section of the Application Configuration guide.
+
 .. _mongoid-version-9.0:
 
 What's New in 9.0