an array where the redundant edges are the indices, and the values are the indices of the first occurence of the duplicate.
Get the indices of all duplicate edges by marking the second/third/... as duplicate (not the first of the duplicates). The result is given as an array with holes, where:
{number[]} array, [4:3, 7:5, 8:3, 12:3, 14:9] where indices
(3, 4, 8, 12) are all duplicates. (5,7), (9,14) are also duplicates.
a FOLD object