Yes. Per RFC 5072, IPv6 over PPP, it says in section 4.1:
4.1. Interface Identifier Description This Configuration Option provides a way to negotiate a unique, 64- bit interface identifier to be used for the address autoconfiguration [3] at the local end of the link (see Section 5). A Configure- Request MUST contain exactly one instance of the interface-identifier option [1]. 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.