Shuffle a sharded iterable dataset without seed can lead to duplicate data
As reported in https://github.com/huggingface/datasets/issues/5360

If `seed=None` in `.shuffle()`, shuffled datasets don't use the same shuffling seed across nodes.

Because of that, the lists of shards is not shuffled the same way across nodes, and therefore some shards may be assigned to multiple nodes instead of exactly one.

This can happen only when you have a number of shards that is a factor of the number of nodes.

The current workaround is to always set a `seed` in `.shuffle()`