It's not uncommon for villagers to have the same name, and it's not this _bug_ unless those villagers are right next to each other in the details screen "rotation" and one of those villagers was 'born' with a different name. Just like I know a bunch of guys named John and Bill, the game often assigns the same name to two different villagers.

There are two ways to have children show a parent with a name of ? - one is the result of banging the gong, and the other is the result of an Island Event. Neither of those are bugs.