diff --git a/docs/source/development/messaging.txt b/docs/source/development/messaging.txt
index 7505acb..6987b0c 100644
--- a/docs/source/development/messaging.txt
+++ b/docs/source/development/messaging.txt
@@ -633,6 +633,10 @@ Message type: ``history_request``::
       # If hist_access_type is 'search', get cells matching the specified glob
       # pattern (with * and ? as wildcards).
       'pattern' : str,
+
+      # If hist_access_type is 'search' and unique is true, do not
+      # include duplicated history.  Default is false.
+      'unique' : bool,
       
     }