DataArray.drop_isel / .drop_sel with duplicated initial time stamp - InvalidIndexError

What is your issue?

I have a DataArray ds with 7098 time steps in cftime format. One of these time steps is a duplicate (pos: 7087 & 7088).
I tried to drop the first instance using both ds.drop_isel(time=[7087]) and ds.drop_sel(ds.time[7087]) but get the error:
pandas.errors.InvalidIndexError: Reindexing only valid with uniquely valued Index objects.
The method works if I do not have the duplicate in it. Maybe this is the wrong method in the first place but it seemed to be the way to go. I am confused on how to get rid of the duplicate, and, if the method is the correct method, if this might be an issue.

ds
Out[247]: 
<xarray.DataArray 'LST_Night_CMG' (time: 3, y: 2, x: 2)>
array([[[14357., 14357.],
        [14342., 14358.]],
       [[14409., 14409.],
        [14435., 14388.]],
       [[14409., 14409.],
        [14435., 14388.]]])
Coordinates:
  * time         (time) object 2019-09-18 00:00:00 ... 2019-09-19 00:00:00
  * x            (x) float64 67.98 68.03
  * y            (y) float64 40.12 40.07
    spatial_ref  int32 0
Attributes:
    coordinates:   spatial_ref band
    scale_factor:  1.0
    add_offset:    0.0


ds.time
Out[249]: 
<xarray.DataArray 'time' (time: 3)>
array([cftime.DatetimeProlepticGregorian(2019, 9, 18, 0, 0, 0, 0, has_year_zero=True),
       cftime.DatetimeProlepticGregorian(2019, 9, 19, 0, 0, 0, 0, has_year_zero=True),
       cftime.DatetimeProlepticGregorian(2019, 9, 19, 0, 0, 0, 0, has_year_zero=True)],
      dtype=object)
Coordinates:
  * time         (time) object 2019-09-18 00:00:00 ... 2019-09-19 00:00:00
    spatial_ref  int32 0



ds.drop_isel(time=1)
Traceback (most recent call last):
  File "C:\Users\einfa\anaconda3\envs\climate\lib\site-packages\IPython\core\interactiveshell.py", line 3444, in run_code
    exec(code_obj, self.user_global_ns, self.user_ns)
  File "<ipython-input-248-0f89b7589e90>", line 1, in <module>
    ds.drop_isel(time=1)
  File "C:\Users\einfa\anaconda3\envs\climate\lib\site-packages\xarray\core\dataarray.py", line 2413, in drop_isel
    dataset = dataset.drop_isel(indexers=indexers, **indexers_kwargs)
  File "C:\Users\einfa\anaconda3\envs\climate\lib\site-packages\xarray\core\dataset.py", line 4565, in drop_isel
    ds = ds.loc[dimension_index]
  File "C:\Users\einfa\anaconda3\envs\climate\lib\site-packages\xarray\core\dataset.py", line 563, in __getitem__
    return self.dataset.sel(key)
  File "C:\Users\einfa\anaconda3\envs\climate\lib\site-packages\xarray\core\dataset.py", line 2505, in sel
    self, indexers=indexers, method=method, tolerance=tolerance
  File "C:\Users\einfa\anaconda3\envs\climate\lib\site-packages\xarray\core\coordinates.py", line 422, in remap_label_indexers
    obj, v_indexers, method=method, tolerance=tolerance
  File "C:\Users\einfa\anaconda3\envs\climate\lib\site-packages\xarray\core\indexing.py", line 120, in remap_label_indexers
    idxr, new_idx = index.query(labels, method=method, tolerance=tolerance)
  File "C:\Users\einfa\anaconda3\envs\climate\lib\site-packages\xarray\core\indexes.py", line 240, in query
    indexer = get_indexer_nd(self.index, label, method, tolerance)
  File "C:\Users\einfa\anaconda3\envs\climate\lib\site-packages\xarray\core\indexes.py", line 142, in get_indexer_nd
    flat_indexer = index.get_indexer(flat_labels, method=method, tolerance=tolerance)
  File "C:\Users\einfa\anaconda3\envs\climate\lib\site-packages\pandas\core\indexes\base.py", line 3442, in get_indexer
    raise InvalidIndexError(self._requires_unique_msg)
pandas.errors.InvalidIndexError: Reindexing only valid with uniquely valued Index objects

Reproduce the failure with the duplicated cftime coordinate, then trace drop_isel through xarray/core/dataarray.py and xarray/core/dataset.py into the indexing code shown in the traceback. Check existing tests for drop_isel or duplicate indexes and add coverage for removing one positional duplicate. Done means the reported operation behaves consistently or the supported workaround is documented by a regression test.