A You are correct in assuming that an 8½-month-old can't remember someone with whom he's had no contact for a year. When your husband returns, there may be some recognition of smells and sounds, but your son will very likely be wary around his dad until he gets comfortable with him again.As you probably know, however, memory is enhanced with practice and repetition. So the trick for you will be to find ways to keep your husband's image and personality easily available in your home, allowing Duncan to have "experiences" with his dad even though he's not physically there. I always say that if Elmo can be real to a child, so can an absent relative.
Start by making some videos of Duncan and your husband together before he leaves on his overseas assignment. They should be videos of the two of them doing fun as well as typical things --having dinner, giving Duncan a bath, going for a ride in the car. Narrate these "Duncan and Daddy" videos with whatever details please your son ("Duncan and Daddy sing a song soooo loudly," "Duncan and Daddy make pancakes --what a mess!"). Make several videos if you can, and show a new segment every few weeks to keep Duncan's interest level high.
Have lots of photos around and point out who Daddy is. Use posterboard and photographs to make a book describing an adventure that Duncan and Daddy had, and then read it together. If nothing else, look at your photo album and talk about the pictures. Make sure to have some pictures of the three of you so he can begin to grasp the concept of you all as a unit. Look at pictures of other families in storybooks and talk about yourselves ("That's like me, you, and Daddy!"). These activities keep the idea of his dad around.
While your husband is away, try to have him send photographs and audiotapes if he can. Looking at and listening to these reminders can provide pleasant opportunities for your son to share an activity with you. If thoughts of your husband make you very sad, however, you might want to have someone else do the sharing: A little maternal sadness is tolerable for children, but a lot is upsetting. If your son sees you get tearful every time you look at the Duncan and Daddy videos, for instance, he may veto them.
It's nice also if your in-laws are around (assuming you have a good relationship with them), because they can offer Duncan an almost visceral connection to their own son. I have no scientific evidence to support this, but I do believe that children feel the links between family members. It might be because of similarities in appearance or manner or scent, or it might be because Grandpa sings "Old MacDonald" the same way your husband does, but being with your husband's parents or siblings may help keep sensations of him lively for your son.