HI - I see a lot of cons and not many pros so thought Id give some of my experiences, not all duplexes are the same.
I live in a duplex - ground floor with just one apt above me.
Pros
Management fee (mine €700) deals with bins (storage shed - no smelly bin in garden), cleans windows once a year, treats wood every other year, covers buildings insurance, fixes any problems that arise outside the building, maintains a very nice landscapped garden.
I have own access front door - drive to front door
I have own pretty large private patio (enough room for garden table for 6 and garden shed) out the back
I have a lovely partially enclosed landscapped garden outside the gated patio shared with another 19 apts that I dont have to tend to. Safe for young children.
Close to neighbours
Larger that the three bed homes in this case. Living room in the largest Ive seen in the areas. Although kitchen fits a table and chairs for only four comfortably
Cons
Can sometimes hear neighbours walking up the stairs (no sounds from apt above)
No attic
Management rules (can be good or bad - no satelite, no hanging out washing)
Cannot change anything about outside area or big jobs inside - no extension in a few years etc.
Hope this helps