The big deal is that when a boundary exists, DRY should be "ignored" (cannot word this decently).
For example, let's say there is an application where a user can purchase items and also give reviews for such purchased items, the user reviewing and the user buying have in common just the ID, while in the review boundary the relevant information is probably the user nickname, while in the purchase boundary the relevant information is the payment system, or the state of the checkout ("purchase" is probably not a single boundary).
In that case, data could be duplicated to ensure the boundaries are decoupled.
This is of course at the data level, but usually it translates to "there is a user model that has many orders and many reviews", because of DRY, no two user models could exist, there you have the boundary violation though.
Sorry, this is a bit of a ramble, it's a long discussion.
For example, let's say there is an application where a user can purchase items and also give reviews for such purchased items, the user reviewing and the user buying have in common just the ID, while in the review boundary the relevant information is probably the user nickname, while in the purchase boundary the relevant information is the payment system, or the state of the checkout ("purchase" is probably not a single boundary).
In that case, data could be duplicated to ensure the boundaries are decoupled.
This is of course at the data level, but usually it translates to "there is a user model that has many orders and many reviews", because of DRY, no two user models could exist, there you have the boundary violation though.
Sorry, this is a bit of a ramble, it's a long discussion.