Minty (1998)

Two teenage girls - Australian television soap star Minty Sullivan, and English schoolgirl Melanie Hobson - are identical in appearance, but not in nature. When Melanie wins a trip to Australia in a competition, and travels to Australia with her father, she and Minty accidentally meet. From then on, the two girls intermittently swap identities. Nobody has ever seen them together, so nobody knows that there are two of them, although some family and friends may be a bit more perceptive than others.