double as; double up as; double back; double over; double up

double as +
兼任 (某職位或某角色)
The actor doubled as the father in the play. (這位演員在劇中兼演父親的角色)

double (up) as +
The kitchen table doubles as my desk when I’m doing my homework. (我在做家庭作業時,這個餐桌還可以當書桌用)
My study doubles up as a spare bedroom when we have visitors. (我們有訪客時,我的書房也兼作備用的臥房)

double back
原路折返 (尤指始料未及者)
When he saw the police, he had to double back. (當他看到警察時,他不得不原路折返)

double over/up
(因痛、笑等而) 彎腰、彎身
John doubled over/up in pain after being hit in the stomach. (約翰肚子挨了一擊後痛得彎下腰)
Amy was doubled over/up with the pain in her stomach. (艾美肚子痛得身子都直不起來了)
The students doubled over/up with laughter at every joke. (這些學生每聽完一個笑話都笑得直不起腰來)

double up
We have only one room left, so you’ll have to double up with Sam. (我們只剩一個房間,所以你必須跟山姆合住一間了)