Does a common task duplicate setup data if I use it in multiple implementation projects?

No, a task used in multiple implementation projects doesn't create duplicate data. Setup data isn't segmented by an implementation project or any other method you use to enter it.

Each task points to the same setup data irrespective of the implementation project from which you perform it. Therefore, when you use a task from a different implementation project, you can also review the existing data and update it if necessary.