Uniqueness

Proabbly a silly question, but here goes.

In the editor for Uniqueness there is No, Unique, Add Once.

What is the difference between Unique and Add Once?

I always thought that a unique item meant it was the only one of its kind so you could only add it once anyway.

I recently had this clarified for me as well, so don't feel bad. Items that are add once can only be user added once. That is, if you take the Endurance feat, you can no longer add it again. However if you then add 3 levels of ranger, another copy comes along (which isn't "user added", even though the user did add the ranger levels). There are separate mechanisms in place to dissuade you from adding "add once" things that have already been bootstrapped by something else (as you'll note if you add 3 ranger levels, and then try to add Endurance as a feat).

In general, we recommend people use Add Once, as Unique can cause issues when combined with Bootstrap Conditions.