Duplica first appeared in "Ditto's Mysterious Mansion." Ash and his friends were traveling through Kanto when they sought shelter from the rain in an abandoned mansion. This house was known as "The House of Imite" and belonged to Duplica and her Ditto. She demonstrated Ditto's ability to transform into any Pokémon. A skeptical Ash sent out his Bulbasaur to battle Duplica's Ditto. Surprisingly, Bulbasaur lost to the imitation. Duplica then explained that her Ditto had difficulty transforming one aspect: its face. Regardless of what Pokémon it transformed into, it always retained the beady eyes and wobbly mouth of a Ditto.
Team Rocket kidnapped the Ditto, intending for it to transform into a Dratini so they could give it to Giovanni. After some training, they managed to get it to transform perfectly. After Ash, Brock, Duplica, and Misty disguised themselves as Team Rocket and rescued Ditto, Duplica felt ready to open her mansion as a theater to entertain passing Pokémon trainers.
In "Imitation Confrontation," Brock noticed that a Nurse Joy they were speaking to was an imitation. It turned out to be Duplica, who had come to town for a Pokémon Acting Competition. She had another Ditto named Mini-Dit, which had a different issue. It could transform into perfect replicas of other Pokémon, except in size. So, when it transformed into an Onix, she ended up with a tiny, hand-held version.
Team Rocket stole Mini-Dit, planning to pass it off as a rare, pygmy Pokémon to trade for a powerful one. Duplica recovered Mini-Dit and decided its unusual transformations made it unique; the audience seemed to agree.
Although Duplica sometimes performs by imitating Pokémon herself, her usual shows feature her imitating a Pokémon Trainer while her Ditto mimics that trainer's Pokémon. For instance, she did an excellent impression of Ash and Nurse Joy.
(Source: Bulbapedia)
