noun
double agent
A spy who pretends to work for one side while actually working for another.
彼は敵国の二重スパイとして活動していた。
He was working as a double agent for the enemy country.
スパイ is a general term for a spy, while 二重スパイ specifically refers to a double agent who serves two opposing sides.
Compound of 二重 (にじゅう, 'double') and スパイ ('spy'), a straightforward modern term.