Given an integer array nums, that can contain duplicate elements,  return all possible subsets while ensuring that each subset is unique. The output must include unique subsets, and you may return them in any order.

Constraints:

• $1 <=$ nums.length $<= 10$

• $-10 <=$ nums[i] $<= 10$

The key idea behind this solution is to use backtracking to explore all possible subsets while carefully avoiding duplicates. We begin by sorting the array, which groups identical values. This is important because it allows us to detect when we’re about to generate a duplicate subset: if two identical elements appear next to each other in the sorted array, we can choose to skip the second one to avoid repetition. After sorting, we run depth-first search (DFS) over the array to build subsets step by step. At every recursion level, we make a sequence of choices: include the current value or move on. To avoid generating duplicate subsets, we skip an element if it is the same as a previously processed value at the same recursion depth. In this way, we explore the complete power set while guaranteeing that no duplicate subset is ever generated.

Using the intuition above, we implement the algorithm as follows:

1. We sort the array, nums, in non-decreasing order.

2. We create an empty array, result, to store all the subsets we generate.

3. We create an empty array, currsubset, to represent the current subset we are building.

4. We perform backtracking, using the helper function backtrack. ...