As the question states, is this bad practice? I have a User aggregate root in the bounded context of Identity for authenticating the user. In this bounded contexts I have fields for the User related to identification of the User e.g. email, salted pw and so on.
I also have a generic subdomain for handling notifications. In this context a User is a Notificant. In this context, the Notificant has fields for e.g. the number of unread notifications, lastRead etc.
Is it good to reuse the User id in this case, as I know there is a 1-to-1 correspondence between a User and Notificant? Or should I have a field in the Notificant root referencing the User? It feels redundant, because then I have to make a lookup to map between them when I know their relationship is symmetric.