once is nonce.

 Einmal ist keinmal (once is nonce: "what happened once might never have happened at all"). Therefore, each life is, ultimately, insignificant; every decision, ultimately, does not matter. Since decisions do not matter, they are light: they do not bind, yet simultaneously, the insignificance of our decisions — our lives, our being — is unbearably light, hence, the unbearable lightness of being.