After the scene where Hallie cut Annies hair she puts the hair she cut on the dresser and pushes the mirror up. Notice that the watch on the wrist that pushes the mirror up has a white band but when Hallie comes into the picture and crosses her arms a few moments later you see that it's a completely differnt watch.
I own the Special Double Trouble Edition which contains special features like bihind the scenes for instance. The producers state that another person was the one that pushed the mirror up. I noticed that since the day that I found out that Lindsay Lohan had played both roles.