-.B --output=summary
-In this case all matching threads are returned but the "match count"
-is the number of matching non-excluded messages in the thread.
+.BR --output=summary .
+The output is almost identical to
+.BR false ,
+but the "match count" is the number of matching non-excluded messages in the
+thread, rather than the number of matching messages.
+.RE
+
+.RS 4
+.TP 4
+.BR \-\-duplicate=N
+
+Effective with
+.BR --output=files ,
+output the Nth filename associated with each message matching the
+query (N is 1-based). If N is greater than the number of files
+associated with the message, don't print anything.
+
+Note that this option is orthogonal with the
+.BR folder:
+search prefix. The prefix matches messages based on filenames. This
+option filters filenames of the matching messages.