Estoy realmente perdido. ¿Por qué me sale NSLog dos veces para cada unoUIImage ?

 //------ get the images from the camera roll ----------
    assets=[[NSMutableArray alloc]init];
    NSMutableArray *cameraRollPictures=[[NSMutableArray alloc]init];
    ALAssetsLibrary *assetsLibrary = [[ALAssetsLibrary alloc] init];
    [assetsLibrary enumerateGroupsWithTypes:ALAssetsGroupSavedPhotos usingBlock:^(ALAssetsGroup *group, BOOL *stop)
    {

        NSInteger numberOfAssets = [group numberOfAssets];
        NSLog(@"NUM OF IMAGES:%d",numberOfAssets);
        if (numberOfAssets > 0)
        {


            for (int i = 0; i <= numberOfAssets-1; i++)
            {

                [group enumerateAssetsAtIndexes:[NSIndexSet indexSetWithIndex:i] options:0 usingBlock:^(ALAsset *result, NSUInteger index, BOOL *stop)
                 {
                    UIImage *thumbnail = [UIImage imageWithCGImage:[result thumbnail]];
                    [assets addObject:thumbnail];
                     NSLog(@"theObject!!!! -- (%d) %@",i,thumbnail);

            //******* for each i its here twice !!   ********

                }];
            }
        }