Let's Scare Jessica to Death

Jessica (Zohra Lampert) has been released from a mental institution to the care of her husband, Duncan (Barton Heyman]), who has given up his job as string bassist for the New York Philharmonic and purchased a farmhouse in Connecticut. When Jessica, Duncan, and their hippy friend Woody (Kevin O'Connor) arrive, they are surprised to find a mysterious drifter, Emily (Mariclare Costello), already living there. When Emily offers to move on, Jessica invites her to dine with them and stay the night.
The following day, Jessica, seeing how attracted Woody is to Emily, asks Duncan to invite her to stay indefinitely. Jessica begins hearing voices and sees a mysterious blonde girl (Gretchen Corbett) looking at her from a distance before disappearing. Later, Jessica is grabbed by someone under the water in the cove where she is swimming. Jessica is afraid to talk about these things with Duncan or Woody, for fear that they'll think she's relapsing. She also becomes aware that Duncan seems to be attracted to Emily, and that the men in nearby town, all of whom are bandaged in some way, are hostile towards them.
Duncan and Jessica decide to sell antiques found in the house at a local shop, one of which is a silver-framed portrait of the house's former owners, the Bishop familyâfather, mother, and daughter Abigail. The antique dealer, Sam Dorker (Alan Manson), tells them the story of how 20-year-old Abigail drowned in 1880 just before her wedding day. Legend says that she's still alive, roving the island as a vampire. Jessica finds the story fascinating, but Duncan, afraid that hearing about such things will upset his wife, cuts Dorker short.
Later, as Jessica prepares to make a headstone rubbing on Abigail Bishop's grave, she notices the blonde girl beckoning her to follow. The girl leads Jessica to a cliff, at the bottom of which lies Dorker's bloodied body. By the time Jessica finds Duncan, however, the body is gone. Jessica and Duncan spot the blonde girl standing on the cliff above them, causing Duncan to give chase. When the girl is caught and questioned by the couple, she remains silent and runs off when Emily approaches.
That night, Duncan tells Jessica that she needs to return to New York to resume her psychiatric treatment. Jessica forces him to sleep on the couch, where he is seduced by Emily.
In the morning, Jessica finds the little mole has been repeatedly stabbed to death. Jessica denies having done it, but Duncan decides that it's time to get help for her and drives into town in search of a phone. When Duncan arrives in town, he goes into a store to attempt to use a telephone, and a number of old men with bandages are seen following in inside.
While Duncan is gone, Jessica goes up to the attic where she is surprised to come face to face with the silver-framed portrait of the Bishop family that they had sold just the previous day to Sam Dorker. She is astounded to see how closely Abigail Bishop resembles Emily.
Suddenly, Jessica sees Emily coming up the stairs. Jessica tries to keep her cool while Emily denies that she is Abigail, all the while she is stroking Jessica's feverish face. Jessica agrees to go with Emily for a cool swim in the lake, even though she's afraid of the water after having been grabbed the other day. As the two women sit at the end of the dock, Emily rubs suntan lotion on Jessica. Suddenly, Emily pushes Jessica into the water and jumps in after her. When Jessica panics, Emily apologizes for scaring her when all she wanted to do was to have a little fun. Jessica wades towards shore and looks back to see that Emily has disappeared. She hears Emily's voice in her head and then sees her reddish-hair bobbing to the surface. When a pale white arm grabs her and tries to pull her underwater, Jessica fights to break loose and return to shore. She looks back at the water and watches Emily, ashen and dressed in Abigail's wedding gown, rising slowly from the water and walking towards her, all the while silently saying, "Stay, Jessica, stay. Follow me."
When Emily reaches Jessica and tries to bite her neck, Jessica breaks free of Emily, and runs back to the house, locks herself in her room, and waits for Duncan to return. Hours pass, during which Jessica can hear Emily's voice whispering to her. When Duncan has not returned by 5:00Â p.m., Jessica changes out of her swimming suit and hitches a ride into town. While Jessica is on her way to town, Woody returns from working the fields to find that the only one at home is Emily. Although Woody doesn't like how Emily has been playing Duncan, he readily submits when she comes on to him. He gets a bite in the neck for his reward.
When Jessica gets into town, she sees Duncan's car and asks about his whereabouts, but none of the men will admit to having seen him. Jessica notices that every man has cuts on his body and backs away...straight into the arms of Sam Dorker. She runs back toward the farm. When she can run no more, she collapses on the ground and lies there until it turns dark and she hears Duncan calling her name. Duncan drives her the rest of the way home, and they go upstairs to bed. Jessica notices a cut on Duncan's neck. Jessica tries to convince herself that it's only in her mind, but then she sees Emily brandishing a carving knife and leading the men from town. Jessica leaps from the bed and runs outside, knocking over Duncan's bass case to see the bloody body of the blonde girl come tumbling out.
Jessica runs through the fields and comes across Woody on a tractor sprayer but when she gets closer, she sees Woody lying on the seat, his neck cut and bloody. At daybreak Jessica makes it to the ferry and tries to board, but the ferryman tells her, "The ferry isn't running for you." Jessica jumps into a nearby rowboat and paddles out into the lake. When a hand reaches into the boat from the water. She stabs the person in the back several times with a long pick. As the body floats away, pale and bloody, she sees that it is Duncan. Back on the shore, Emily/Abigail and the men from town can be seen watching her.
"I sit here, and I can't believe that it happened," Jessica says to herself in voice over, "and yet I have to believe it. Nightmares or dreams? Madness or sanity? I don't know which is which."

Question: Who does Jessica stab?
Answer: Duncan