Return an object that maps each value appearing more than once in arr to the positions where it appears. Positions are zero-based and listed in increasing order. Values that appear only once are left out.
Example 1
Input
arr = [1, 2, 2, 3, 3, 3, 4]
Output
{ 2: [1, 2], 3: [3, 4, 5] }
Explanation
2 sits at positions 1 and 2, and 3 at positions 3, 4 and 5.
Example 2
Input
arr = [5, 5, 5, 5]
Output
{ 5: [0, 1, 2, 3] }
Explanation
one value fills every position.
Example 3
Input
arr = ["a", "b", "a", "c", "b", "b"]
Output
{ a: [0, 2], b: [1, 4, 5] }
Explanation
the values can be text as well. "c" appears once, so it is left out.
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