That doesn't guarantee unique values. With the built-in rand/srand, it produces repeats within 30 picks for seeds: 

seed : no.of picks before repeat
89 : 12
178 : 17
485 : 9
504 : 14
555 : 22
662 : 16
688 : 23
761 : 19
766 : 18
920 : 26
936 : 25
943 : 19
955 : 17
982 : 14
1004 : 27
1032 : 19
1192 : 23
1200 : 9
1385 : 24
1427 : 17
1439 : 29
1459 : 22
1582 : 14
1593 : 22
1602 : 29
1668 : 12
1737 : 22
1924 : 29
1933 : 24
1969 : 18
2186 : 27
2261 : 12
2297 : 12
2380 : 18
2410 : 29
2660 : 25
2690 : 28
2733 : 24
2817 : 13
2863 : 4
2875 : 19
2964 : 28
3023 : 27
3052 : 9
3198 : 24
3208 : 23
3240 : 28
3292 : 16
3415 : 2
3529 : 26
3669 : 22
3692 : 29
3741 : 21
3754 : 13
3791 : 23
3805 : 28
3842 : 24
3854 : 8
3956 : 28
4077 : 27
4202 : 20
-- More  --

[download]

Testcase:

#! perl -slw
use strict;

for my $i ( 1 .. 100000 ) {
    my %hash;
    srand $i;
    ++$hash{ int rand 100000 } == 2 and print "$i : ", scalar keys %ha
+sh
        while keys %hash < 30;
}

[download]
With the rise and rise of 'Social' network sites: 'Computers are making people easier to use everyday'
Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
"Science is about questioning the status quo. Questioning authority".
In the absence of evidence, opinion is indistinguishable from prejudice.

In reply to Re^2: Non-Repetitive Random Numbers by BrowserUk
in thread Non-Repetitive Random Numbers by Anonymous Monk

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