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.