YOU SAID:
We shall double our efforts.
INTO JAPANESE
倍の努力をしなければなりません。
BACK INTO ENGLISH
You must double efforts.
INTO JAPANESE
二重努力をする必要があります。
BACK INTO ENGLISH
You need to double efforts.
INTO JAPANESE
努力を倍増する必要があります。
BACK INTO ENGLISH
You need to redouble their efforts.
INTO JAPANESE
彼らの努力を倍加する必要があります。
BACK INTO ENGLISH
You need to redouble their efforts.
Come on, you can do better than that.