#ifndef's before including the headers (redundant inclusion test?)

[Nuno Antunes]

Hi.

Why is the following test necessary before including headers? (for
example in sys/netgraph/netgraph.h).
#ifndef _SYS_QUEUE_H_
#include <sys/queue.h>
#endif
when <sys/queue.h> already tests for inclusion?

#ifndef _SYS_QUEUE_H_
#define _SYS_QUEUE_H_
. ..
#endif
Thanks,
Nuno