fix double word error;

src/lib/libc/string/strncat.3

@@ -1,4 +1,4 @@
-.\"	$OpenBSD: strncat.3,v 1.1 2013/12/19 20:52:37 millert Exp $
+.\"	$OpenBSD: strncat.3,v 1.2 2013/12/19 22:00:58 jmc Exp $
 .\"
 .\" Copyright (c) 1990, 1991 The Regents of the University of California.
 .\" All rights reserved.
@@ -124,5 +124,5 @@ functions conform to
 .Sh HISTORY
 The
 .Fn strncat
-function first first appeared in
+function first appeared in
 .At v7 .