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。