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:

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

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