Jan Murphy was a woman that had a resemblance to Corrine Mackey.
Jan Murphy was meeting with Steven Linfield at a motel several times. When Vic Mackey hired Gordie Liman to find his wife and kids, Gordie sent Corrine's picture to several motels and other places where she might be. When the motel manager where Jan was staying misidentified Jan as Corrine he called Gordie, telling him the woman in the picture was staying in one of the rooms.
When Vic met in the motel parking with Gordie, he asked him in what room she was. Gordie told him that a man was in the room with her. Vic then stormed the room and found Jan having sex with Steve. He threw Steve away only to realize she wasn't Corrine and left.