Error with repeated dimension name in DataTree

[TomNicholas]

This seems like a real error, even if it is the edge case of two identically-named dimensions...

>           assert_identical(unaligned_dict_of_datasets["/Group1"], expected)
E           AssertionError: Left and right Dataset objects are not identical
E           Differing dimensions:
E               (lat: 1, lon: 2) != (lon: 2)
E           Differing data variables:
E           L   group_1_var  (lat, lon) float64 16B ...
E           R   group_1_var  (lon, lon) float64 32B ...

Originally posted by @TomNicholas in #10139 (comment)