Bug in network clustering: cannot insert bus1_s, already exists

[davide-f]

Checklist

• I am using the current master branch or the latest release. Please indicate:

Describe the Bug

This problem arises in the function get_buses_linemap_and_lines in the following code when the lines dataframe is empty:

return (
        buses,
        linemap,
        linemap_p,
        linemap_n,
        lines.reset_index()
        .rename(columns={"bus0_s": "bus0", "bus1_s": "bus1"}, copy=False)
        .set_index("name"),
        lines_t,
    )

This seems to be due to the function aggregatelines called previously that does stuff on the lines dataframe including groupbys.
That should normally drop such columns but that does not happen when the dataframe is empty.
This is an error we experienced when running the pypsa-earth workflow on rwanda.

Error Message

If applicable, paste any terminal output to help illustrating your problem.
In some cases it may also be useful to share your list of installed packages: conda list.

Exception has occurred: ValueError       (note: full exception trace is shown but execution is paused at: _run_module_as_main)
cannot insert bus1_s, already exists
  File "/home/davidef/miniconda3/envs/pypsa-earth/lib/python3.10/site-packages/pandas/core/frame.py", line 4814, in insert
    raise ValueError(f"cannot insert {column}, already exists")
  File "/home/davidef/miniconda3/envs/pypsa-earth/lib/python3.10/site-packages/pandas/core/frame.py", line 6358, in reset_index
    new_obj.insert(
  File "/home/davidef/miniconda3/envs/pypsa-earth/lib/python3.10/site-packages/pandas/util/_decorators.py", line 331, in wrapper
    return func(*args, **kwargs)
  File "/home/davidef/miniconda3/envs/pypsa-earth/lib/python3.10/site-packages/pypsa/networkclustering.py", line 351, in get_buses_linemap_and_lines
    lines.reset_index()
  File "/home/davidef/miniconda3/envs/pypsa-earth/lib/python3.10/site-packages/pypsa/networkclustering.py", line 378, in get_clustering_from_busmap
    buses, linemap, linemap_p, linemap_n, lines, lines_t = get_buses_linemap_and_lines(
  File "/data/davidef/git_world/pypsa-earth/scripts/cluster_network.py", line 444, in clustering_for_n_clusters
    clustering = get_clustering_from_busmap(
  File "/data/davidef/git_world/pypsa-earth/scripts/cluster_network.py", line 577, in <module>
    clustering = clustering_for_n_clusters(
  File "/home/davidef/miniconda3/envs/pypsa-earth/lib/python3.10/runpy.py", line 86, in _run_code
    exec(code, run_globals)
  File "/home/davidef/miniconda3/envs/pypsa-earth/lib/python3.10/runpy.py", line 196, in _run_module_as_main (Current frame)
    return _run_code(code, main_globals, None,

[pz-max]

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