unkinkPolygon cant deal with duplicate coordinates

This is basically a re-opening of:

• #1679

As that was closed by the author without any fix 6 years ago.

unkinkPolygon throws an error The input polygon may not have duplicate vertices (except for the first and last vertex of each ring) when run on a polygon with duplicate vertices.

Reproduction using an hourglass shape, where the center has 2 stacked vertices:

const hourglass = turf.polygon( [[
    [0,0],
    [1,1], // center
    [2,2],
    [0,2],
    [1,1], // center
    [2,0],
    [0,0]
]]);

// all of these error
turf.unkinkPolygon(hourglass)
turf.unkinkPolygon(turf.cleanCoords(hourglass))
turf.unkinkPolygon(turf.simplify(hourglass, {tolerance: 0.5}))
turf.unkinkPolygon(turf.simplify(hourglass, {tolerance: 0.5, highQuality: true}))

Start at the unkinkPolygon entry point and run the supplied hourglass reproduction, including the variants using cleanCoords and simplify. Trace where duplicate vertices are rejected and define the expected result for this self-intersecting polygon; done means the reproduction no longer throws solely because of the stacked center coordinates.