Sameness of personality (8)
I believe the answer is:
identity
This clue is a double definition.
'sameness' is the first definition.
(I know that identity is a type of sameness)
'personality' is the second definition.
(I know that identity is a type of personality)
'of' is the link.
(Other definitions for identity that I've seen before include "Close affinity" , "papers" , "A person's individuality" , "Individuality providing recognition" , "Individuality or singularity" .)