"The Double-Dip Horror" is a short story in More & More & More Tales to Give You Goosebumps.
Twin sisters Rachel and Wynona have arrived at the Ice Cream Cone Ski Lodge for a week. They were hired to be junior instructors with the deal being that they teach for six days and on the seventh day, they get to ski for free. The twin sisters, realizing that no one has seen both of them, formulate a plan. They'll both take shifts being only Rachel. When one twin is being Rachel and teaching lessons, the other twin will be skiing for free. The plan works generally well, except for a red-haired kid named Bobby Judd, who keeps making trouble for the twins whenever they try to teach the class. The other students ignore his antics and the twins do their best to follow suit. Bobby Judd acts up but is actually a very good skater and keeps begging the twins for a private lesson.
One of the twins comes up with the perfect plan. They'll show Bobby Judd up by agreeing to a lesson. Then they'll take turns skiing down the same mountain, always coming out ahead of Bobby no matter what. Bobby Judd won't be able to beat them because they'll be able to cheat and trade places and he'll finally be put in his place. Bobby agrees to the plan, but insists on meeting them on a black diamond course, the Double-Dip. The real Rachel starts the race and Wynona waits for Bobby to show up so she can slide ahead, but he never does. Finally she goes looking for Rachel and both sisters discover that neither has seen Bobby. Worried that he might be lost, Wynona agrees to ski down to the lodge to see if anyone has heard from Bobby while Rachel waits on the slope.
At the lodge, Wynona asks Margot, the head ski instructor, if she's seen Bobby. Margot thinking Wynona is Rachel, tells her there is no student named Bobby. Wynona insists that there is a Bobby Judd and describes him. Margot gets very serious and tell her that Bobby Judd was the son of a ski instructor who died on a ski course. He haunts the lodge and tries to lure kids into racing with him. Apparently a murdering ghost is something you causally forget to mention to potential junior ski instructors. Wynona tells Margot that they have to do something, as her friend is up on the course, waiting to race the ghost. Margot tells her not to worry, the ghost only tries to murder identical twins.