YOU SAID:

Oh no that's not the first time I've done that!

INTO JAPANESE

いや、そんなことしたのは初めてじゃないよ！

BACK INTO ENGLISH

No, it's not the first time I've done that!

INTO JAPANESE

いいえ、それをしたのは初めてではありません!

BACK INTO ENGLISH

No, it's not the first time I've done that!