Dynamic polygons on terrain error with duplicate positions

Reported on the forum: https://groups.google.com/forum/#!topic/cesium-dev/KV_HKOCrBWs

Code to duplicate:
```
var viewer = new Cesium.Viewer('cesiumContainer', {
terrainProvider: Cesium.createWorldTerrain()
});

var positions = Cesium.Cartesian3.unpackArray([
178357.7526681642, -3986013.8412960134, 4936566.6195444185,
-2354798.5072417534, -4447471.461059317, 3874377.1963103525,
898401.3668786548, -5541508.785762887, 2964274.437825734,
928843.2226594514, -4515751.1774108, 4376108.315712495,
928843.2226594514, -4515751.1774108, 4376108.315712495
]);

viewer.entities.add({
polygon: {
hierarchy: new Cesium.CallbackProperty(function (time, result) {
return positions;
}, false),
outline: true,
material: new Cesium.ColorMaterialProperty(Cesium.Color.WHITE.withAlpha(0.7)),
}
});
```

Start by running the supplied Cesium.Viewer reproduction with createWorldTerrain and the entity polygon hierarchy using CallbackProperty. Trace the polygon terrain path from viewer.entities.add and determine how duplicate final positions produce the reported error. Done means the reproduction no longer errors with those positions, with a regression test added wherever the repository’s existing polygon tests cover this path.

Written by the indexing model from the issue text.