use Tie::IxHash;
tie my %group, Tie::IxHash;

## group them by ID, preserving the order seen

for (@records) {
    my $id = $_->{ID};
    push @{ $group{$id} }, $_;
}

## put special header at beginning of groups larger than 1

my $last_id;
while (my ($curr_id, $records) = each %group) {
    unshift @$records, { ... } if @$records > 1;
    $last_id = $curr_id;
}

## flatten everything out

my @complete_list = map @$_, values %group;