YOU SAID:

He must not have checked his list twice like he's supposed to.

INTO JAPANESE

彼は自分のリストを2回チェックしてはいけませんでした。

BACK INTO ENGLISH

He did not check his list twice.

INTO JAPANESE

彼は自分のリストを二度チェックしなかった。

BACK INTO ENGLISH

He did not check his list twice.