The Peanuts (ザ・ピーナッツ, Za Pīnattsu) were a Japanese vocal duo comprising the twin sisters Emi (伊藤エミ, Itō Emi) and Yumi Itō (伊藤ユミ, Itō Yumi). They were born in Nagoya on April 1, 1941. Their voices were close in timbre, which resulted in their singing together sounding like a solo artist using double tracking or reverb.
Source: “Yumi Ito”, English Wikipedia (CC BY-SA 4.0), excerpt