Date range field with unlimited cardinality duplicates on edit of entity

Thanks for the report and the patch, but I am afraid the fix is not 100% correct.

The defaultDate for me is always the same. So when I actually set two dates then on edit both date will be the same, even though item.value contains the right value.

I think the issue is somewhere in the DateTimeRangeFlatPickrWidget formElement method,
that $element['value']['#attached']['drupalSettings']['datetimeFlatPickr'][$name]
the $name is likely the same for each item in the multivalue form so the settings are overriden.

So I think the proper fix is just 1 line actually.

Bit more lines to make tests pass after that.