Mum: 'Do your homework (now), Aneta!' Aneta: "I've done it (already)!" talking later: Aneta: "my mum told me to do my homework (earlier) but I'd already done it" ('have done' is about now, 'have done' is the same thing but in a past time)

I have done means again there is a possibility to do it again. But I had done it means never I do it again.

I have done it，it means I have done it already . I had done it, it mean that you had done something before. They have the different tense .

I believe the difference is the tense. " I have done it" is present perfect tense. "I had done it" is past perfect tense. Why? Perfect tense always uses the verb in its past form, i.e. done instead of do or doing. Present perfect uses the present form of the words "have" or "has" + past participle (done). Past perfect uses the past form of have or has, which is "had" + past participle (done). Future Perfect will therefore use "will + have" and the past participle (done).

They are absolutely different. We use "have done" in case we talk about something completed recently or nowadays. We use "had done" when we talk about something completed a long time ago in the past.