Until this gets implemented you can manually edit the MDX and just wrap the Distinct() function around the entire set. If there are any duplicates then only the first instance will be kept.
However, be aware that the Distinct function works on members NOT elements. Duplicate members with the same MUN will be removed but if you have duplicate elements which are identified with different MUNs due to a different rollup path these won’t be seen as duplicates (becasue from a MDX perspective they aren’t.)
If this is wrapped in Distinct there will still be 2 members in the set as there are 2 unique rollup paths to the element 2023.
Regarding removing duplicates in Arc Subset Editor, rather than selecting and removing duplicates, sometimes I just select the members I want to keep (including one of the duplicate members) and use the Keep button. This, of course, only makes sense with smaller sets.