is Reference proxy must be unique

Is reference proxy must be unique ,

I've been a handled an app which has reference proxy field & not unique ,

Actually , The reference proxy for the grand child is the same as the parent
means that :-
 A- In  1-m relation ship between (grandfather& parent) reference proxy field in unique in the grandfather table. That's correct 

B - The second 1-M relationship (parent & grand child ) reference proxy is the same field used in the previous relationship. ( Not unique & it used in the higher 1-M relationship)  Here is the paradox 

I feel that is not correct at all ,Right ? See the below image to explain




------------------------------
Yasser Sabry
------------------------------