diff --git a/src/flag/flag.go b/src/flag/flag.go
index 9fed4d82b3..6a51617524 100644
--- a/src/flag/flag.go
+++ b/src/flag/flag.go
@@ -314,6 +314,9 @@ const (
 
 // A FlagSet represents a set of defined flags. The zero value of a FlagSet
 // has no name and has ContinueOnError error handling.
+//
+// Flag names must be unique within a FlagSet. An attempt to define a flag whose
+// name is already in use will cause a panic.
 type FlagSet struct {
 	// Usage is the function called when an error occurs while parsing flags.
 	// The field is a function (not a method) that may be changed to point to