This bug was reported for the "View mode" UI 🐛 Duplicate "Add view mode" buttons RTBC but actually applies to "Form mode" as well.
The /admin/structure/display-modes/form page has multiple ways to add form modes. Some users were confused about what's the difference between the two options.
In usability testing, vast majority of users navigate to this page from "Manage Display" and "Manage Form Display". From this perspective, we could remove the buttons that are specific to an entity type after #2721727: Allow user to add display modes from respective field UI's. has been done.