BUG: areal weighting methods produce erroneous results when there exists duplicate source geoms/unique identifiers

[tastatham]

Methods that interpolate values based on areal weighting (as well as pycno) produce erroneous results when there are are duplicate geometries.
Geometries may be duplicated when handing certain types of datasets e.g. survey data.
In this scenario, I was using survey data and I simply forgot to perform a groupby per unique identifier, prior to running the interpolation.
This returned interpolated values for all targets but the results were erroneous.

My initial thought was to update all areal_checks to add an additional check for duplicate geometries/unique identifiers;

coss/coss/interpolation.py

Line 65 in 3d8274d

def areal_checks(self, method):

However, some area interpolation methods may produce "better" estimates when there exists multiple values per geometry or unique identifier e.g statistical ones.

My idea is to add a check, to see whether duplicate values per geometry/unique identifier are present.