The popular cartoon adventure time has many doubles so I’m gonna try and knock em out as many as I can:
Finn and Fern:
Finn Murtens, the protagonist of the show has himself be imbued into a sword, so that it can truly be an extension of himself. But when a cursed plant imbues itself in the sword it creates Fern. Although at first they seem identical, and have the same memories, the clashing of differences between the current Finn, the Finn which Fern came from, and Fern himself, has such an impact that Fern is driven to a hatred of himself. And from the hatred of himself comes the hatred of Finn.
Simon Petrikov and the Ice King:
Simon was a professor in history from centuries in the past who comes in contact with a magical crown. This crown transforms whomever wears it into the Ice King, twisting the original wearer over time until they’re unrecognizable. The two are the same person yet also are so different you have to see them separately. Yet the small similarities in the Ice King constantly pokes at the people who miss Simon.
Jake and Jermaine:
Identical brothers, living extremely different lives with extremely different views on their father. Jake was treated specially thanks to his abilities while Jermaine often had to work much harder for affection. But as the years pass after his fathers death the two become closer now that there is no competing.
Bona-belle and Uncle Gumbald:
Bona-belle created her own Uncle(along with an aunt and cousin) out of her own DNA because she was alone. But just like she doesn’t want to control her family like a ruler, her Uncle wants to be a ruler and not part of a Family. In the end, neither of them get what they want, and Bona-belle becomes the monarch of a Candy Kingdom she was forced to make from a brainwashed family.
The Litch and Sweet Pea:
In a show with alternate realities, the largest antagonist of the show(the Litch) has a fight with his reborn self(Sweet Pea). The two come from the same primordial embodiment of death and chaos, but while the Litch remains a paragon of this, Sweet Pea is the opposite: A young child who’s growing and learning from better examples.
Magic Man and Betty Groff
The title of “Magic Person” is given through irreparable loss, a refusal to accept reality, and of course, magic. Betty is the successor of magic man, who both lose their loved ones to cosmic horrors and try to use their amazing powers to bring them back, never succeeding. It is also from these fruitless attempts we see these Magic People lose empathy for others, forming a sort of tunnel vision to their goals.
BMO and Football
Football is an alternate persona to the friendly(albeit childish) BMO, who lives in their reflection. While at times Football feels more like an imaginary friend, the two sometimes are at odds over who gets to live outside of their reflection. It’s a bizarre blurring of having an alter ego, an imaginary friend, and multiple personalities.
The two Lemon Grabs
Lemon Grab 1 was built by Bona-belle differently from almost anyone, having strange tastes and overly sensitive. This left him feeling angry frustrated and isolated, until Bona-belle built a second Lemon Grab. The two build their Lemon kingdom, but soon end up fighting between kindness and bitterness. Lemon Grab 1 goes so far as to eat Lemon Grab 2 to have full control over the kingdom, although Lemon Grab 2 later escapes.
Sam's Two English Cents 