Always used with a negative verb form to express that something will not happen a second time. Stronger and more emphatic than a simple negative.
あんな怖い思いは二度としたくない。
I never want to go through such a scary experience again.
He never came back again.
I will never see him again.
I will never go to that shop again.
I will never repeat the same mistake again.
I never want to make that kind of mistake again.