If you own a duplex house, have you ever rented out to two tenants who ended up hating each other after living next to each other? I know a lot of people who have lived in a duplex house and had to deal with close neighbors who made their lives hell sometimes. It's why I think it's hard to rent duplexes out, because often what happens, is that the new tenant will end up getting into it with an old tenant or vice versa.
