Fix items bug of dataset that doesn't support items

[boeddeker]

Fix for cache, from_dataset and new to work properly with input datasets, that have no support for keys, because they are duplicated.

The issue was, that duplicates were dropped:

• list(ds.items()) worked and created a list of key values pairs
• dict(list(ds.items())) removed duplicated keys.

Now the code checks, that no examples get lost and concatenate is fixed to raise an exception, when items is called, while some keys are duplicated.

[boeddeker]

Now the code checks, that no examples get lost and concatenate is fixed to raise an exception, when items is called, while some keys are duplicated.

Concatenate doesn't know if a following filter may remove duplicates. Hence, added back the items support in concatenate and check instead in from_dataset, if the input dataset has duplicates.