The Seven Potters

Ron, Hermione, Fred, George, Fleur and Mundungus drank. All of them gasped and grimaced as the Portion hit their throats: at once, their features began to bubble and distort like hot wax. Hermione and Mundungus were shooting upwards; Ron, Fred and George were shrinking; their hair was darkening, Hermione's and Fleur's appearing to shoot backwards into their shulls.
...
Fred and George turned to each other and said together, 'Wow- we're identical'

(Harry Potter and the Deathly Hallows, The Seven Potters