Hi Bart,
As a default in Reactive apps, the Login screen is in the Template module, which means that this screen is cloned every time you create a Reactive application.
If you want your applications to share a login screen, you can create a Style Guide where you have the Login screen in the Theme module instead of in the Template modules.
Here is a link of a lesson that really helped me to understand this:
https://www.outsystems.com/training/lesson/1235/base-style-guide-architecture?LearningPathId=17
Let me know if this was clear,
Pedro