let PiecewiseLinearMap::new return error if input mappings contain duplicate keys

[anthrotype]

slice::binary_search isn't well defined with multiple values #933

In #935 (comment) I commented

I now also wonder if PiecewiseLinearMap::new (and as a result CoordConverter::new) should be fallible, in case we ever construct them with duplicate from inputs which would lead to a similar ambiguous situation.

in fonttools this situation never occurs because mappings are a dict where keys are unique by definition.
When reading a designspace axis' maps, the list of (user, design) tuples is turned into a dict (with a dict comprehension, where last key wins in case of duplicates):
https://github.com/fonttools/fonttools/blob/c2119229cfb02cdb7c5a63374ef29d3d514259e8/Lib/fontTools/designspaceLib/__init__.py#L1031-L1047

not sure if fontmake warns about this ambiguity (maybe glyphsLib does when constructing the DS axes)