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
1046454702,I_kwDOAMm_X84-X6Gu,5944,Backend / plugin system `remove_duplicates` raises AttributeError on discovering duplicates,9155111,closed,0,,,11,2021-11-06T07:46:38Z,2021-11-15T17:33:43Z,2021-11-15T17:33:43Z,CONTRIBUTOR,,,,"<!-- Please include a self-contained copy-pastable example that generates the issue if possible.

Please be concise with code posted. See guidelines below on how to provide a good bug report:

- Craft Minimal Bug Reports: http://matthewrocklin.com/blog/work/2018/02/28/minimal-bug-reports
- Minimal Complete Verifiable Examples: https://stackoverflow.com/help/mcve

Bug reports that follow these guidelines are easier to diagnose, and so are often handled much more quickly.
-->

**What happened**:

In [one of my CI runs](https://github.com/eX-Mech/pymech/runs/4124742207?check_suite_focus=true#step:7:248) somehow the entrypoints ended up being defined twice. Then, I discovered that the function `remove_duplicates` which weeds out duplicate entrypoints were not updated.

**What you expected to happen**:

No bugs in `remove_duplicates` function

**Minimal Complete Verifiable Example**:

```python
# Put your MCVE code here
from xarray.backends.plugins import remove_duplicates
from importlib.metadata import entry_points

eps = entry_points().get('xarray.backends', ())

remove_duplicates(eps)
remove_duplicates(eps + eps)
```

```
<ipython-input-12-22df5e55614a>:1: DeprecationWarning: EntryPoints list interface is deprecated. Cast to list if needed.
  remove_duplicates(eps + eps)
---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-12-22df5e55614a> in <module>
----> 1 remove_duplicates(eps + eps)

~/.pyenv/versions/3.9.7/envs/pymech/lib/python3.9/site-packages/xarray/backends/plugins.py in remove_duplicates(entrypoints)
     27         matches_len = len(matches)
     28         if matches_len > 1:
---> 29             selected_module_name = matches[0].module_name
     30             all_module_names = [e.module_name for e in matches]
     31             warnings.warn(

AttributeError: 'EntryPoint' object has no attribute 'module_name'
```

**Anything else we need to know?**:

Following v0.20.0 the entrypoints were discovered using `importlib.metadata` / `importlib_metadata`, but it was broken for 3rd party backeds. In v0.20.1 after #5931 the backend detection was fixed, but remains to be thoroughly tested #5934. This bug might be rare, compared to #5930, so I would recommend having some tests in place before making the next release. 

**Environment**:

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

<!-- Paste the output here xr.show_versions() here -->

INSTALLED VERSIONS
------------------
commit: None
python: 3.9.7 (default, Nov  3 2021, 09:51:04) 
[GCC 11.1.0]
python-bits: 64
OS: Linux
OS-release: 5.10.75-1-lts
machine: x86_64
processor: 
byteorder: little
LC_ALL: None
LANG: C
LOCALE: (None, None)
libhdf5: 1.12.1
libnetcdf: None

xarray: 0.20.1
pandas: 1.3.4
numpy: 1.21.3
scipy: None
netCDF4: None
pydap: None
h5netcdf: 0.11.0
h5py: 3.5.0
Nio: None
zarr: None
cftime: None
nc_time_axis: None
PseudoNetCDF: None
rasterio: 1.2.10
cfgrib: None
iris: None
bottleneck: None
dask: 2021.10.0
distributed: None
matplotlib: None
cartopy: None
seaborn: None
numbagg: None
fsspec: 2021.10.1
cupy: None
pint: None
sparse: None
setuptools: 57.4.0
pip: 21.3.1
conda: None
pytest: 6.2.5
IPython: 7.29.0
sphinx: 4.2.0


</details>
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1043276928,I_kwDOAMm_X84-LySA,5930,Regression: 3rd party backends are not discovered with `xarray==0.20.0`,9155111,closed,0,,,7,2021-11-03T09:25:48Z,2021-11-11T10:06:47Z,2021-11-03T16:55:55Z,CONTRIBUTOR,,,,"**What happened**:
I am a library developer for `pymech` and I added a (unreleased) xarray backend following the documentation and `rioxarray`'s implementation. This works with `xarray==0.19.0`, but with the `xarray==0.20.0` it no longer works. Could someone clarify what has changed?

**What you expected to happen**:

Backends can be discovered while using `xr.open_dataset(...)` with or without using the `engine=` keyword argument.

**Minimal Complete Verifiable Example**:

Since `pymech`'s xarray backend is unreleased, I will demonstrate it via rioxarray:

```py
#test.py
from xarray.backends import plugins

print(""engines ="", plugins.list_engines())
plugins.get_backend(""rasterio"")
```

```sh
$ pip install xarray==0.19.0 rioxarray
$ python test.py
engines = {'rasterio': <rioxarray.xarray_plugin.RasterioBackend object at 0x7f7adca15d00>, 'store': <xarray.backends.store.StoreBackendEntrypoint object at 0x7f7adca647c0>}

$ pip install xarray==0.20.0
$ python test.py
engines = {'store': <xarray.backends.store.StoreBackendEntrypoint object at 0x7fd99138efa0>}
Traceback (most recent call last):
  File ""<string>"", line 1, in <module>
  File ""/tmp/venv/lib/python3.9/site-packages/xarray/backends/plugins.py"", line 164, in get_backend
    raise ValueError(
ValueError: unrecognized engine rasterio must be one of: ['store']

```

**Anything else we need to know?**:

**Environment**:

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


INSTALLED VERSIONS
------------------
commit: None
python: 3.9.7 (default, Oct 10 2021, 15:13:22) 
[GCC 11.1.0]
python-bits: 64
OS: Linux
OS-release: 5.10.75-1-lts
machine: x86_64
processor: 
byteorder: little
LC_ALL: None
LANG: C
LOCALE: (None, None)
libhdf5: None
libnetcdf: None

xarray: 0.20.0
pandas: 1.3.4
numpy: 1.21.3
scipy: None
netCDF4: None
pydap: None
h5netcdf: None
h5py: None
Nio: None
zarr: None
cftime: None
nc_time_axis: None
PseudoNetCDF: None
rasterio: 1.2.10
cfgrib: None
iris: None
bottleneck: None
dask: None
distributed: None
matplotlib: None
cartopy: None
seaborn: None
numbagg: None
fsspec: None
cupy: None
pint: None
sparse: None
setuptools: 57.4.0
pip: 21.2.3
conda: None
pytest: None
IPython: 7.29.0
sphinx: None



</details>
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