What's the difference in upgrade behavior between standard layouts and duplicated layouts?

After an upgrade, the standard layouts for an object's creation and details pages automatically include any new underlying changes made as part of the upgrade.

However, duplicated layouts (copied versions of standard layouts) exist independently from the original, standard layout, and in general, do not uptake most underlying changes that come with an upgrade. In other words, duplicated layouts remain functionally identical to their preupgrade versions, even after an upgrade.

Thus, after every upgrade, carefully review all your duplicated layouts. You can re-create them if you want to pick up new changes introduced in the upgrade.