살다살다 something을/를 처음본다 is used like one sentence. It means in my whole life i never seen this before. So when you saw something that surprised or shocked you, you can use it if you never seen that before. So I want to say 살다살다 is in my whole life.

살다 살다 is short for 살다가 살다가. "X-다가 Y" means Y happens unexpectedly while doing X (운동하다가 발을 다쳤다 = I hurt my foot while exercising), so 살다가 X는 처음 봅니다 means "I've seen X for the first time in my life", like it's something very unusual and rare so that you'd have to bring up your entire life instead of mere months or years. If you repeat it, the meaning is strengthened, like "I've seen so many things in my long life, but this still surprises me". The same form sometimes have a slightly different meaning of "no matter how (one) tried", expressing how long or hard something has been. For example, - 살다 살다 저렇게 막돼 먹은 사람은 처음이다 = This is the first time in my whole life I've seen such a bad mannered guy. - 하다 하다 안돼서 포기했어 = 해도 해도 안돼서 포기했어 = I gave up after failing no matter how hard I tried.