Fixes #5841

• [X] Closes #5841
• [ ] Tests added
• [X] Passes pre-commit run --all-files
• [ ] User visible changes (including notable bug fixes) are documented in whats-new.rst
• [ ] New functions/methods are listed in api.rst

What happened:

In one of my CI runs 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:

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:

See #2888

Criteria for inclusion: - Mentioned in https://xarray.pydata.org/en/stable/installing.html#optional-dependencies - Listed in PyPI - Should not fail during build (due to build dependencies or external libraries)

• [x] Closes #2888
• [x] User visible changes (including notable bug fixes) are documented in whats-new.rst