lodash find duplicate element index

1const _ = require('lodash');
2let arr = ["a", "a", "b", "c", "c", "a"];
3
4let result = _.omitBy(
5  _.reduce(arr, (a, v, i) => _.set(a, v, (a[v] || []).concat([i])), {} ),
6  v => v.length <= 1
7);
8
9console.log(result);