You can use the same skill for different entities, but you can't define different skills with the same name.

This is possible, because the engine can't miss the right skill:
#define health skill1
#define stamina skill1

This is not possible, because the name "health" doesn't tell the engine which of the two skills has to be used:
#define health skill1
#define health skill2

That's at least the way I understand how it works.

EDIT:
The second part could be wrong.
Maybe, it works this way, and the problem begins only when using a second define for one of the skills.

Let's say one entity uses these defines:
#define health skill1
#define stamina skill2

but there is a define for another entity like this:
#define health skill2

then the engine changes both skills when changing the value of 'health' because the define doesn't whose skill is meant.