Double is the stage name for Japanese pop singer Takako Hirasawa. Previously, it was the name for a vocal group comprising Takako and her sister Sachiko. Takako and Sachiko Hirasawa were born in the Niigata prefecture in Japan, and made their debut as a duo in 1998 under the name "Double." Sachiko died in 1999. Takako, rather than abandoning the name, adopted it as her stage name. Wikipedia
The definitive Wikipedia entry for Double. Wikipedia is the biggest multilingual free-content encyclopedia on the Internet. en.wikipedia.org/wiki/Double_%28singer%29