As suggested by Ammal i changed the code so this is the current version:

-(NSDictionary *)groupUsingBlock:(id<NSCopying> (^)(id obj))block {
    NSMutableDictionary *dictionary = [NSMutableDictionary dictionary];
    
    for (id obj in self) {
        id<NSCopying> key = block(obj);
        if (! dictionary[key]) {
            NSMutableArray *arr = [NSMutableArray array];
            dictionary[key] = arr;
        }
        [dictionary[key] addObject:obj];
    }
    return [dictionary copy];
}

I've tryed even a concurrent version, but unfortunately doesn't works

-(NSDictionary *)groupUsingBlockOptimized:(id<NSCopying> (^)(id obj))block {
    NSMutableDictionary *dictionary = [NSMutableDictionary dictionary];
    
    
    [self enumerateObjectsWithOptions:NSEnumerationConcurrent
                           usingBlock:^(id obj, NSUInteger idx, BOOL *stop){
                               id<NSCopying> key = block(obj);
                               if (! dictionary[key]) {
                                   NSMutableArray *arr = [NSMutableArray array];
                                   dictionary[key] = arr;
                               }
                               [dictionary[key] addObject:obj];
                           }];
    return [dictionary copy];
    
}