List get lowest position z when branches have same x,y

[Joseph_Oster]

Separate Lists by Z Coordinate when number per sublist varies

This example of MIndex finds points with the same X and Y values, differing only in Z, but you can easily adapt it to find duplicate Z values: [MIndex_2017Jan22a] There can be precision issues that require rounding, as explained here:

Set Member Index Example (MIndex)