It seems to me that your database design (or the name of the 'id' column) is broken if you have non-unique ID's.

As for supporting this in the module: I'm not inclined to support something in DBIx::Simple that makes the value type returned dependent on the contents of the result set. That results in rather fragile code. You could just add a method, though:

# untested

package DBIx::Simple::Result::Metaperl;
our @ISA = qw(DBIx::Simple::Result);

sub map_arrays_of_hashes {
    my ($self, $column_name) = @_;
    my %arrays;
    for my $row ($self->hashes) {
        push @{ $arrays{$row->{id}} }, $row;
    }
    return wantarray ? %arrays : \%arrays;
}

...

$db->result_class = 'DBIx::Simple::Result::Metaperl';

my %arrays_of_hashes = $db->query(...)->map_arrays_of_hashes('id');
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To keep DBIx::Simple simple, I'm not adding a proliferation of variants; a vast number map_foos_of_bars permutations could be thought of and I'm sure there's they could all be useful in someone's code somewhere, but let's draw a line... :)