Duplication is a bug that can happen sometimes when returning from travel, basically, "traveling" is saving the world, "exporting" the family to a new neighborhood, and importing them back when travel is over. What happens is that he didn't got properly removed from the original neighborhood in the first place, glitching things when importing him back.
Clones often create great ravages through neighborhoods, relationships, for example, get wonky, and so do family trees; if you were unlucky, this corrupted data could spread and eventually lead to a glitched/unplayable neighborhood.
This happens when it's created however, so leaving him around or deleting him won't make the slightest difference. So, enjoy the ride and hope for the best.
However, if you haven't saved since it happened, you might want to go to your saves folder and make a backup of it (particularly, a backup of the backup save files). These could be used to return the neighborhood to it's previous, hopefully unspoiled state if worse comes to happen.
