The worst part is that the initial requirements are often way closer to something you build before, but "small adjustments" make it into a completely different thing.
I agree 100%. The requirements might sound the same, but only until you dig in.
I've often received the same requirements for a system that already existed, and had to ask, why do you need a new one instead of the one you already have that does exactly what you're asking for? And oh boy does the answer to that get interesting.
> So often we're asked to build something we've built before, so we estimate with confidence that it might take a week.
I‘ve never built the same thing / received the same requirements twice, making estimation pointless.