that在這裏是指income, 收入。
double有動詞的用法
柯林斯字典的解釋：V-ERG 及物/不及物動詞(使)加倍;（使）增加壹倍 When something doubles or when you double it, it becomes twice as great in number, amount, or size.
例如：The program will double the amount of money available to help pay for child care. 該方案將使可用於幫助支付兒童保育的錢款增加壹倍。