Check for duplicate axis label when coercing measure

[jbousquin]

Specifically the error is:
ValueError: cannot reindex on an axis with duplicate labels

And comes up for GOM Phosphorus in _coerce_measure():
df_out[self.out_col] = meas_s

It occurs where geopandas is trying to re-index, this geopandas version has a relevant deprecation warning:
~lib\site-packages\geopandas\geodataframe.py:1443: FutureWarning: reindexing with a non-unique Index is deprecated and will raise in a future version.