Memories and Vampyrs - Dark Journeys Series, #5

When Judd Denning first meets Seda on the beach in Southern California after his discharge from the US Navy, she tells him that the word "vampire" should really be "vampyr": and pronounced vam-peer.  Many years later, he learns that she not only still exists but has a double-walker or doppelganger named Lucia. His therapist in the present-day Atlanta does not believe this, of course, and blames his mental condition for his insistance in her existence.
Lucia, however, turns out to be viscious regarding his friends nearby and back in his home state of Illinois. When he learns about the existence of an Asian type of vampyr known as an aswang, he consults and works with a former student from the Philippines. This creature, he learns, can also produce doppelgangers but is even harder to spot during the day than a vampyr. Lives must be saved and sanity must be regained, but success with one does necessarily mean success with the other.