Double are defined as 2 different bands, one from Japan, the other from Switzerland.
DOUBLE (Japan) originally started with two members, sisters SACHIKO and TAKAKO. TAKAKO was born into a Niigata family as the youngest of three sisters who got along famously and were always singing together. (TAKAKO does some rare reflecting on this period in an interview on her recently released video, or DVD, "DOUBLE".) As is the case with most families, it was the oldest sibling who wielded the greatest influence on her younger sisters, SACHIKO and TAKAKO.