id,node_id,number,title,user,state,locked,assignee,milestone,comments,created_at,updated_at,closed_at,author_association,active_lock_reason,draft,pull_request,body,reactions,performed_via_github_app,state_reason,repo,type
614976784,MDU6SXNzdWU2MTQ5NzY3ODQ=,4049,to_unstacked_dataset broken for single-dim variables,530988,closed,0,,,0,2020-05-08T20:43:39Z,2020-07-02T20:51:11Z,2020-07-02T20:51:11Z,CONTRIBUTOR,,,,"<!-- A short summary of the issue, if appropriate -->


#### MCVE Code Sample

```python
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

<details><summary>Output of <tt>xr.show_versions()</tt></summary>

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


</details>
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611384793,MDExOlB1bGxSZXF1ZXN0NDEyNTg0NDI5,4021,Fix to_netcdf docstring typo,530988,closed,0,,,1,2020-05-03T10:57:20Z,2020-05-03T13:34:27Z,2020-05-03T13:34:27Z,CONTRIBUTOR,,0,pydata/xarray/pulls/4021,,"{""url"": ""https://api.github.com/repos/pydata/xarray/issues/4021/reactions"", ""total_count"": 0, ""+1"": 0, ""-1"": 0, ""laugh"": 0, ""hooray"": 0, ""confused"": 0, ""heart"": 0, ""rocket"": 0, ""eyes"": 0}",,,13221727,pull