my @twice = do {
	my %tracker;
	foreach my $a_ref (\@a1, \@a2, \@a3) {
		foreach my $a_value (@$a_ref) { 
			$tracker{$a_value}{$a_ref} = undef;
		}
	}
	grep {2 == keys %{$tracker{$_}}} keys %tracker;
};