Document allow_choice_duplicates (#1924)

Author: grzesiek2010

docs/xlsform.rst

@@ -67,6 +67,7 @@ Other available columns are:
 - ``public_key``: This attribute is necessary for enabling :ref:`encryption <defining-encrypted-form>`. It represents a base64-encoded RSA public key. The corresponding private key will be required for decrypting submissions and should not be included in the form definition.
 - ``auto_send``: When set to true, any finalized forms will be automatically sent as soon as a connection becomes available. If present, it will override the app-level :ref:`Auto send <auto-send>` setting.
 - ``auto_delete``: When set to true, successfully submitted forms will be immediately deleted from the device. If present, it will override the app-level :ref:`Delete after send <delete-after-send>` setting.
+- ``allow_choice_duplicates``: When set to ``yes``, enables the use of duplicate choice names within the same list. This is sometimes necessary, for example, in cascading selects where choices with the same name are distinguished by a preceding question. Without this setting, forms with duplicate choice names will produce an error and not convert.
 
 .. _instance-name: