Insert Delete GetRandom O(1) - Duplicates allowed

Store all occurrences in an array and map each value to a set of indices where it occurs. Insert appends the value and adds the new index to its set; it returns whether the set was previously empty if the API wants a boolean. For remove, take any index from the target value's index set, identify the array's last value and index, move that last value into the removed slot, then update the moved value's set by deleting the old last index and adding the new slot. Pop the array and delete the target map key only when its set becomes empty. If target and last value are the same, order set updates carefully so the selected occurrence is not reintroduced incorrectly. getRandom remains uniform over occurrences because it samples the array index. Test one value repeated many times, removal of last element, target equal to last, and final removal of a key. Expected time is O(1) per operation and space O(n).