Dataset length is twice the number of images used

There are 16 pictures in val directory, len(dls.valid_ds) is 32.
Same happens with train, with 5216 -> 10432.
Code I’m using:

dblock = DataBlock(blocks = (ImageBlock, CategoryBlock),
                  get_items = get_image_files,
                  get_y = parent_label,
                  splitter = GrandparentSplitter(train_name='train', valid_name='val'),
                  item_tfms = Resize(224))

dls = dblock.dataloaders(path)

What’s causing this? I assume this is the expected behaviour, I’d like to understand what’s going on.

Thanks a lot!