diff --git a/man2/kcmp.2 b/man2/kcmp.2
index 9ef9736ac..31c84f281 100644
--- a/man2/kcmp.2
+++ b/man2/kcmp.2
@@ -75,6 +75,13 @@ as file descriptor
 .I idx2
 in the process
 .IR pid2 .
+The existence of two file descriptors that refer to the same
+open file description can occur as a result of
+.BR dup (2)
+(and similar)
+.BR fork (2),
+or passing file descriptors via a domain socket (see
+.BR unix (7)).
 .TP
 .BR KCMP_FILES
 Check whether the processes share the same set of open file descriptors.