$collection = Model::where()->select('id', 'customer_id', 'season_id')->get();

$customers = Model::select('customer_id')->get()->toArray();
$arr_customer_id = array_flatten($customers); // массив всех customer_id

$seasons = Model::select('season_id')->get()->toArray();
$arr_season_id = array_flatten($seasons); // массив всех season_id


$collection->each(function ($item, $key) {
    
    if ( in_array($item, $arr_customer_id) ) {
        // код ...
    }
    
    if ( in_array($item, $arr_season_id) ) {
        // код ...
    }
});

Model::groupBy('customer_id', 'season_id')->havingRaw('COUNT(*) > 1')->get()

Model::all('id', 'customer_id', 'season_id')->unique(static function($item): bool {
    return $item['customer_id'].$item['season_id'];
});