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
557257598,MDU6SXNzdWU1NTcyNTc1OTg=,3731,Repeated coordinates leads to unintuitive (broken?) indexing behaviour,8238804,open,0,,,13,2020-01-30T04:21:57Z,2023-11-30T08:58:22Z,,NONE,,,,"#### MCVE Code Sample
<!-- In order for the maintainers to efficiently understand and prioritize issues, we ask you post a ""Minimal, Complete and Verifiable Example"" (MCVE): http://matthewrocklin.com/blog/work/2018/02/28/minimal-bug-reports -->

```python
import xarray as xr
import numpy as np

sample_idx = xr.IndexVariable(""sample_id"", [""a"", ""b"", ""c""])
da = xr.DataArray(np.eye(3), coords=(sample_idx, sample_idx))

da.shape
# (3, 3)
da[1, :].shape
# (3, 3)
da.loc[""a"", :].shape
# (3, 3)
da.loc[:, ""a""].shape
# ()
da[:, 0].shape 
# ()
da[:, 1]
# <xarray.DataArray ()>
# array(1.)
# Coordinates:
#    sample_id  <U1 'b'
```

#### Expected Output

I had expected:

```python
da.shape
# (3, 3)
da[1, :].shape
# (3)
da.loc[""a"", :].shape
# (3)
da.loc[:, ""a""].shape
# (3)
da[:, 1]
# <xarray.DataArray (sample_id: 3)>
# array([0., 1., 0.])
# Coordinates:
#    sample_id  <U1 'a' 'b' 'c'
```

#### Problem Description

When coordinates are shared between dimensions (as would happen if a pairwise measurement is taken) indexing behaves strangely. It looks like indexing into the initial indices doesn't do anything, while indexing into the last index applies the selection across all dimensions.

```python
da3d = xr.DataArray(
    np.arange(27).reshape((3,3,3)),
    coords=(sample_idx, sample_idx, sample_idx)
)

print(da3d.loc[""a""].shape)
print(da3d.loc[""a"", ""a""].shape)
print(da3d.loc[:, :, ""a""].shape)
# (3, 3, 3)
# (3, 3, 3)
# ()
```

#### Output of ``xr.show_versions()``
<details>
# Paste the output here xr.show_versions() here

```
INSTALLED VERSIONS
------------------
commit: None
python: 3.7.6 (default, Jan  4 2020, 12:18:30) 
[Clang 11.0.0 (clang-1100.0.33.16)]
python-bits: 64
OS: Darwin
OS-release: 19.2.0
machine: x86_64
processor: i386
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8
LOCALE: en_US.UTF-8
libhdf5: 1.10.2
libnetcdf: 4.6.3

xarray: 0.14.1+5.gb0064b25
pandas: 0.25.3
numpy: 1.18.1
scipy: 1.4.1
netCDF4: 1.5.2
pydap: None
h5netcdf: 0.7.4
h5py: 2.9.0
Nio: None
zarr: 2.4.0
cftime: 1.0.3.4
nc_time_axis: None
PseudoNetCDF: None
rasterio: None
cfgrib: None
iris: None
bottleneck: None
dask: 2.9.2
distributed: 2.9.3
matplotlib: 3.0.3
cartopy: None
seaborn: 0.10.0
numbagg: None
setuptools: 44.0.0
pip: 20.0.2
conda: None
pytest: 5.3.4
IPython: 7.11.1
sphinx: 2.3.1
```

</details>


Update, adding link to related issue:

*  https://github.com/pydata/xarray/issues/1378","{""url"": ""https://api.github.com/repos/pydata/xarray/issues/3731/reactions"", ""total_count"": 1, ""+1"": 1, ""-1"": 0, ""laugh"": 0, ""hooray"": 0, ""confused"": 0, ""heart"": 0, ""rocket"": 0, ""eyes"": 0}",,,13221727,issue