I felt disappointed to find that I hadn't done anything, compared to what those geniuses, such as Newton and Einstein, had done.

Does the last part have to be the past perfect? Is it possible for the sentence to have the simple past there instead?
TakaDoes the last part have to be the past perfect? Is it possible for the sentence to have the simple past there instead?
Yes, the simple past is possible, but the past perfect is better. It emphasizes that Newton and Einstein's work preceded the work of the speaker.
I felt disappointed to find that I hadn't done anything, compared to what those geniuses, such as Newton and Einstein, had done.

Here we have 'hadn't done' before 'had done', and before 'hadn't done' we have 'felt'.

So when a sentence has 'past→past perfect1→past perfect2', does it mean 'past→past further back→past still further back'?
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That's the way I interpret it.

Certainly the action of "hadn't done" precedes the action "feeling".

Unfortunately, there is no past-past verb form to show an even earlier action, so the interpretation is from context.
I see. So such double past perfect usage doesn't sound redundant to your native ear?
TakaI see. So such double past perfect usage doesn't sound redundant to your native ear?
No, it is very common and entirely natural. I automatically infer the past/older past temporal relationship.
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simple past tense is almost always preferrable: "taka felt disappointed to find that i didn't do anything, compared to what those geniuses .... did.