YOU SAID:

You've haven't not done this before, haven't you not.

INTO JAPANESE

あなたは前にこれをしたことがありませんね。

BACK INTO ENGLISH

You've never done this before, have you?

INTO JAPANESE

あなたはこれまでにこれをしたことがありませんよね？

BACK INTO ENGLISH

You've never done this before, have you?