Yes. Per RFC 5072, IPv6 over PPP, it says in section 4.1:
4.1. Interface Identifier
This Configuration Option provides a way to negotiate a unique, 64-
bit interface identifier to be used for the address autoconfiguration
 at the local end of the link (see Section 5). A Configure-
Request MUST contain exactly one instance of the interface-identifier
option . The interface identifier MUST be unique within the PPP
link; i.e., upon completion of the negotiation, different interface-
identifier values are to be selected for the ends of the PPP link.
The interface identifier may also be unique over a broader scope.