diff --git a/man7/epoll.7 b/man7/epoll.7
index 535ba03d2..49150fb4e 100644
--- a/man7/epoll.7
+++ b/man7/epoll.7
@@ -239,9 +239,19 @@ What happens if you add the same file descriptor to an epoll set twice?
 .B A1
 You will probably get
 .BR EEXIST .
-However, it is possible that two
-threads may add the same file descriptor twice.
-This is a harmless condition.
+However, it is possible to add a duplicate
+.RB ( dup (2),
+.BR dup2 (2),
+.BR fcntl (2)
+.BR F_DUPFD ,
+.BR fork (2))
+descriptor to the same
+.B epoll
+set.
+This can be a useful technique for filtering events,
+if the duplicate file descriptors are registered with different
+.I events
+masks.
 .TP
 .B Q2
 Can two