Make reuse message more clear

flag.go

diff --git a/flag.go b/flag.go
index 743e011..e828e48 100644
--- a/flag.go
+++ b/flag.go
@@ -868,7 +868,7 @@
 	c := shorthand[0]
 	old, alreadythere := f.shorthands[c]
 	if alreadythere {
-		fmt.Fprintf(f.out(), "%s shorthand reused: %q for %s and %s\n", f.name, c, name, old.Name)
+		fmt.Fprintf(f.out(), "%s shorthand reused: %q for %s already used for %s\n", f.name, c, name, old.Name)
 		panic("shorthand redefinition")
 	}
 	f.shorthands[c] = flag