The original 1961 movie The Parent Trap was far superior to the 1998 remake. You can fight me on that. The special effects of the Lindsey Lohan version may have been better, but the earlier movie was more believable, because the entire idea of separating identical twins and keeping the other a secret was repugnant by 1998.
Okay, back in 1961, a lot of people really thought that Hayley Mills was a twin, because how could she do this movie otherwise? Her twin was played by Susan Henning, who was only a year younger and looked quite a bit like Mills. They were even the same size! We mostly saw the back of Henning's head in the film, but when you see the girls together offscreen, you realize they could have gotten away with showing more of her. She also worked as a stand-in for the scenes that were stitched together. Henning was not credited for her work, because that might have spoiled the suspension of disbelief. Henning wasn't upset about that, neither then nor 60 years later, because she got to be a part of Disney magic.