Document the behavior of dup2 when oldfd == newfd: nothing happens.

Suggested by the patches in PR bin/36551 from Greg A. Woods.

lib/libc/sys/dup.2

@@ -1,4 +1,4 @@
-.\"	$NetBSD: dup.2,v 1.18 2004/05/13 10:20:58 wiz Exp $
+.\"	$NetBSD: dup.2,v 1.19 2008/02/09 04:58:30 dholland Exp $
 .\"
 .\" Copyright (c) 1980, 1991, 1993
 .\"	The Regents of the University of California.  All rights reserved.
@@ -29,7 +29,7 @@
 .\"
 .\"     @(#)dup.2	8.1 (Berkeley) 6/4/93
 .\"
-.Dd June 4, 1993
+.Dd February 8, 2008
 .Dt DUP 2
 .Os
 .Sh NAME
@@ -95,6 +95,11 @@ If this descriptor is already
 in use, the descriptor is first deallocated as if a
 .Xr close 2
 call had been done first.
+If
+.Fa newd
+and
+.Fa oldd
+are the same, the call has no effect.
 .Sh RETURN VALUES
 The value \-1 is returned if an error occurs in either call.
 The external variable