Lookalike

A look-alike is a person who closely resembles another person. Appearance may be the only connection between the two people, as they may not be related, or their personalities may be quite different.

Examples

 * Thaddeus and Thor and Phineas and Ferb — Similar in appearance, but opposite in personalities. ("Thaddeus and Thor")
 * Jeremy and Charles Pipping IV — Similar in appearance. ("Flop Starz", "A Hard Day's Knight")
 * Ferb and Ferbgor — Distant relatives nearly identical in appearance. ("Rollercoaster", "The Monster of Phineas-n-Ferbenstein").
 * Isabella and the Isabella (lookalike) — Similar to attitude and sweetness and same to say the line "Whatcha' doin?" ("Rollercoaster", "The Monster of Phineas-n-Ferbenstein")
 * Candace and Constance — They both are identical and want to bust their relatives.
 * Phineas and Dr. Phineastein — Have the same optimism and attitude, although Dr. Phineastein has a more evil/insane personality.
 * Jeremy and Jeremiah — They both look alike, and Jeremy likes Candace in the same way Jeremiah likes Constance.
 * Heinz Doofenshmirtz and Jekyll Doofenshmirtz — They are not only related, but are also alike in attitude, always adding something to their inventions, such as a self-destruct button, or a fairy princess dial. They also aspire to be evil, but usually fail.
 * Roger Doofenshmirtz and Jameson — They appear to be related since Jameson may be the ancestor of Roger, except for the fact that Jameson worked as a butler for Jekyll, who is an ancestor of Heinz, who is the brother of Roger. They also look alike.
 * Perry the Platypus and Unnamed robe-wearing platypus — They are both anthropomorphic platypuses.
 * Perry the Platypus and Platypus monster — They make almost the same sound and fought against their nemesis.
 * Betty Jo and Lorraine —They are identical twins. ("Get That Bigfoot Outa My Face!")
 * Candace and Princess Baldegoon - They look nearly identical, but they have no relation. ("Make Play")