use strict;
use warnings;
use Hash::MultiValue;
use 5.012;

my $mhash = Hash::MultiValue->new(
        "one" => "3",
        "two" => "2",
        "two" => "1"
);

die "Duplicate key detected for \%hash" if grep { my @a = $mhash->get_all($_); @a > 1 } $mhash->keys;

print "We're good!";