to_unstacked_dataset broken for single-dim variables

[letmaik]

MCVE Code Sample

arr = xr.DataArray(
     np.arange(3),
     coords=[("x", [0, 1, 2])],
 )
data = xr.Dataset({"a": arr, "b": arr})
stacked = data.to_stacked_array('y', sample_dims=['x'])
unstacked = stacked.to_unstacked_dataset('y')
# MergeError: conflicting values for variable 'y' on objects to be combined. You can skip this check by specifying compat='override'.

Expected Output

A working roundtrip.

Problem Description

I need to stack a bunch of variables and later unstack them again, however this doesn't work if the variables only have a single dimension.

Versions

Output of xr.show_versions()

INSTALLED VERSIONS

commit: None
python: 3.7.3 (default, Mar 27 2019, 22:11:17)
[GCC 7.3.0]
python-bits: 64
OS: Linux
OS-release: 4.15.0-96-generic
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: en_GB.UTF-8
LOCALE: en_GB.UTF-8
libhdf5: 1.10.4
libnetcdf: 4.6.2

xarray: 0.15.1
pandas: 1.0.3
numpy: 1.17.3
scipy: 1.3.1
netCDF4: 1.4.2
pydap: None
h5netcdf: None
h5py: 2.10.0
Nio: None
zarr: None
cftime: 1.0.4.2
nc_time_axis: None
PseudoNetCDF: None
rasterio: None
cfgrib: None
iris: None
bottleneck: None
dask: 2.10.1
distributed: 2.10.0
matplotlib: 3.1.1
cartopy: None
seaborn: 0.10.0
numbagg: None
setuptools: 41.0.0
pip: 19.0.3
conda: 4.8.3
pytest: 5.3.5
IPython: 7.9.0
sphinx: None