List every distinct subset of numbers with repeats

A test harness runs one case for every combination of the input values it was given. Values can repeat, and it should never run the same combination twice.

Write subsets_with_dup(nums) that returns every distinct subset of nums, in a fixed order.

Input and output:

• nums: a list of up to 12 integers, each between -100 and 100. Values may repeat.
• Returns a list of lists of integers.

Rules:

• A subset takes any number of the values, from none to all. Each value can be used at most as many times as it appears in nums.
• Two subsets are the same if they hold the same values the same number of times. Return each distinct subset once.
• The empty subset is included.
• Each subset is sorted in ascending order.
• Sort the list of subsets by length, shortest first. Subsets of the same length are ordered by their first value, then their second, and so on, comparing numbers by value.