[ns] Why does the RNG seed change ?

[syjsyjsyj]

Hello,
I try to produce a random set of 5 DIFFERENT integers in [0 21], using the 
following code:
set rng [new RNG]
$rng seed 1
for {set i 0} {$i < 5} {incr i 1} {
 puts "seed [$rng seed]"
 set node [$rng integer 21]
 puts "random integer $node"
}
Output:
seed 1
random integer 11
seed 1
random integer 20
seed 1
random integer 12
seed 592852
random integer 12
seed 592852
random integer 17
I don't understang why the seed changes ->integer overlapping (12 appears two 
time)
Thanks in advance