my @old_arr = ( 'foo', 'bar', 'zoot', 'boo', 'bar', 'zoot' ) ;
my %hash = map { $_ => 1 } @old_arr ;
my @new_arr = keys %hash ;

my $dups_found = @old_arr - @new_arr ;
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Originally posted as a Categorized Answer.

Another way.....in constant (long) time.

#! perl -w
use Quantum::Superpositions;
use strict;
local ($,=", ", $\="\n");
my @ary = qw/ 1 2 3 3 4 5 6 6 7 8 9/;
my @unique = eigenstates(any(@ary));
print @ary;
print @unique;

my @dStrings = qw/ alpha alpha bravo charlie delta echo foxtrot charli
+e golf golf golf hotel india/;
my @uStrings = eigenstates(any(@dStrings));
print @dStrings;
print @uStrings;

my @chars = eigenstates(any(split //, "the quick brown fox jumps over 
+the lazy dog"));
print sort @chars;
__DATA__
C:\test>181519
1, 2, 3, 3, 4, 5, 6, 6, 7, 8, 9
1, 2, 3, 4, 5, 6, 7, 8, 9
alpha, alpha, bravo, charlie, delta, echo, foxtrot, charlie, golf, gol
+f, golf, hotel,
 india
bravo, charlie, india, golf, delta, hotel, alpha, echo, foxtrot
 , a, b, c, d, e, f, g, h, i, j, k, l, m, n, o, p, q, r, s, t, u, v, w
+, x, y, z
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Originally posted as a Categorized Answer.