Listing /proc/PID/task were PID is not a TGID should not result in
duplicated entries.

	[g ~]$ pidof thunderbird-bin
	2751
	[g ~]$ ls /proc/2751/task
	2751  2770  2771  2824  2826  2834  2835  2851  2853
	[g ~]$ ls /proc/2770/task
	2751  2770  2771  2824  2826  2834  2835  2851  2853
	2770  2771  2824  2826  2834  2835  2851  2853
	[g ~]$
Signed-off-by: NGuillaume Chazarain <guichaz@yahoo.fr>
Acked-by: N"Eric W. Biederman" <ebiederm@xmission.com>
Cc: Oleg Nesterov <oleg@tv-sign.ru>
Signed-off-by: NAndrew Morton <akpm@osdl.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>