diff --git a/man2/select.2 b/man2/select.2
index d061c2475..01f59c42c 100644
--- a/man2/select.2
+++ b/man2/select.2
@@ -125,12 +125,16 @@ This macro adds the file descriptor
 .I fd
 to
 .IR set .
+Adding a file descriptor that is already present in the set is a no-op,
+and does not produce an error.
 .TP
 .BR FD_CLR ()
 This macro removes the file descriptor
 .I fd
 from
 .IR set .
+Removing a file descriptor that is not present in the set is a no-op,
+and does not produce an error.
 .TP
 .BR FD_ISSET ()
 .BR select ()