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
422903285,MDU6SXNzdWU0MjI5MDMyODU=,2822,to_netcdf() fails to write if input is read from netcdf3_classic,5186647,closed,0,,,6,2019-03-19T19:05:01Z,2023-12-15T20:48:05Z,2023-12-15T20:48:04Z,NONE,,,,"#### Code Sample

```python
import xarray as xr
with xr.open_dataset('some_netcdf_classic_file') as ds:
    data = ds
    #do something with data
#Neither of these work, see exception below
data.to_netcdf('some_new_file', 'w')
data.to_netcdf('some_new_file', 'a')
#Writing back to the same file, or a new file with mode of NETCDF3_CLASSIC works
data.to_netcdf('some_netcdf_classic_file', 'a')
data.to_netcdf('some_new_file', 'w', 'NETCDF3_CLASSIC')
```
#### Problem description
```
  File ""/home/nels.frazier/.local/lib/python2.7/site-packages/xarray/core/dataset.py"", line 1232, in to_netcdf
    compute=compute)
  File ""/home/nels.frazier/.local/lib/python2.7/site-packages/xarray/backends/api.py"", line 747, in to_netcdf
    unlimited_dims=unlimited_dims)
  File ""/home/nels.frazier/.local/lib/python2.7/site-packages/xarray/backends/api.py"", line 790, in dump_to_store
    unlimited_dims=unlimited_dims)
  File ""/home/nels.frazier/.local/lib/python2.7/site-packages/xarray/backends/common.py"", line 263, in store
    self.set_attributes(attributes)
  File ""/home/nels.frazier/.local/lib/python2.7/site-packages/xarray/backends/common.py"", line 279, in set_attributes
    self.set_attribute(k, v)
  File ""/home/nels.frazier/.local/lib/python2.7/site-packages/xarray/backends/netCDF4_.py"", line 421, in set_attribute
    _set_nc_attribute(self.ds, key, value)
  File ""/home/nels.frazier/.local/lib/python2.7/site-packages/xarray/backends/netCDF4_.py"", line 297, in _set_nc_attribute
    obj.setncattr(key, value)
  File ""netCDF4/_netCDF4.pyx"", line 2619, in netCDF4._netCDF4.Dataset.setncattr
  File ""netCDF4/_netCDF4.pyx"", line 1479, in netCDF4._netCDF4._set_att
  File ""netCDF4/_netCDF4.pyx"", line 1745, in netCDF4._netCDF4._ensure_nc_success
AttributeError: NetCDF: String match to name in use
```
The generated attribute error isn't clear at all, the file actually gets created, but then fails in the processes of writing.  There is no clear connection to the netCDF format.

#### Expected Output
A new file created/written in (default) NETCDF4 format with contents of the Dataset.  Or at the very least provide an exception that clearly indicates the format incompatibilities.

<details>

#### Output of ``xr.show_versions()``
INSTALLED VERSIONS
------------------
commit: None
python: 2.7.5 (default, Sep 12 2018, 05:31:16)
[GCC 4.8.5 20150623 (Red Hat 4.8.5-36)]
python-bits: 64
OS: Linux
OS-release: 3.10.0-957.5.1.el7.x86_64
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8
LOCALE: None.None
libhdf5: 1.8.18
libnetcdf: 4.4.1.1

xarray: 0.11.3
pandas: 0.22.0
numpy: 1.14.2
scipy: 1.0.0
netCDF4: 1.4.1
pydap: None
h5netcdf: None
h5py: 2.7.1
Nio: None
zarr: None
cftime: 1.0.1
PseudonetCDF: None
rasterio: None
cfgrib: None
iris: None
bottleneck: None
cyordereddict: None
dask: None
distributed: None
matplotlib: 1.2.0
cartopy: None
seaborn: None
setuptools: 40.4.3
pip: 18.1
conda: None
pytest: None
IPython: None
sphinx: None
</details>
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