Duplicate Supports for Persistent Buff Skills - Intended/Bugged?

Currently, some of the Supports for Spirit Gems/Persistent Skills can be slotted multiple times into different skills (as of the Abyssals update, this is I think?expected behavior). For example Thornskin 2 (40 Spirit Reservation; 70% increased Thorns damage while supported skill is active) can be placed into as many Persistent Skills as resources allow.

Vitality 2 (40 Spirit Reservation; regenerate 2% Max Life/second), on the other hand, cannot have multiple copies on different Persistent buffs.

What's the design intent? Should these Spirit supports be stackable? Is it a fluke that some can have multiple copies active? Does the des ription for the Vitality support line miss some exclusive qualifier?