ValueError: cannot reindex from a duplicate axis (could use better error)

[mortonjt]

I've been playing around with CTF and have occasionally noticed the following error

Traceback (most recent call last):
  File "/home/juermieboop/miniconda3/envs/qiime2-2020.8/lib/python3.6/site-packages/q2cli/commands.py", line 329, in __call__
    results = action(**arguments)
  File "<decorator-gen-538>", line 2, in ctf
  File "/home/juermieboop/miniconda3/envs/qiime2-2020.8/lib/python3.6/site-packages/qiime2/sdk/action.py", line 245, in bound_callable
    output_types, provenance)
  File "/home/juermieboop/miniconda3/envs/qiime2-2020.8/lib/python3.6/site-packages/qiime2/sdk/action.py", line 390, in _callable_executor_
    output_views = self._callable(**view_args)
  File "/home/juermieboop/miniconda3/envs/qiime2-2020.8/lib/python3.6/site-packages/gemelli/ctf.py", line 40, in ctf
    feature_metadata)
  File "/home/juermieboop/miniconda3/envs/qiime2-2020.8/lib/python3.6/site-packages/gemelli/ctf.py", line 187, in ctf_helper
    straj = concat([straj.reindex(all_sample_metadata.index),
  File "/home/juermieboop/miniconda3/envs/qiime2-2020.8/lib/python3.6/site-packages/pandas/util/_decorators.py", line 221, in wrapper
    return func(*args, **kwargs)
  File "/home/juermieboop/miniconda3/envs/qiime2-2020.8/lib/python3.6/site-packages/pandas/core/frame.py", line 3976, in reindex
    return super().reindex(**kwargs)
  File "/home/juermieboop/miniconda3/envs/qiime2-2020.8/lib/python3.6/site-packages/pandas/core/generic.py", line 4514, in reindex
    axes, level, limit, tolerance, method, fill_value, copy
  File "/home/juermieboop/miniconda3/envs/qiime2-2020.8/lib/python3.6/site-packages/pandas/core/frame.py", line 3864, in _reindex_axes
    index, method, copy, level, fill_value, limit, tolerance
  File "/home/juermieboop/miniconda3/envs/qiime2-2020.8/lib/python3.6/site-packages/pandas/core/frame.py", line 3886, in _reindex_index
    allow_dups=False,
  File "/home/juermieboop/miniconda3/envs/qiime2-2020.8/lib/python3.6/site-packages/pandas/core/generic.py", line 4577, in _reindex_with_indexers
    copy=copy,
  File "/home/juermieboop/miniconda3/envs/qiime2-2020.8/lib/python3.6/site-packages/pandas/core/internals/managers.py", line 1251, in reindex_indexer
    self.axes[axis]._can_reindex(indexer)
  File "/home/juermieboop/miniconda3/envs/qiime2-2020.8/lib/python3.6/site-packages/pandas/core/indexes/base.py", line 3362, in _can_reindex
    raise ValueError("cannot reindex from a duplicate axis")
ValueError: cannot reindex from a duplicate axis

Turns out that the error pops up if there are duplicates in the state-column, and the error goes away of these duplicates are removed (i.e. if week 3 is repeated twice within the same host, it'll throw an error).

It could be nice if a more informative error was thrown.