But the thing is, in software engineering - if you ever need to do again, something you have done before, it should be just a copy away.
So you'll never spend time on something you've done before. And if you do, that time is essentially wasted - something that shouldn't be billable.
Now that's the ideal, obviously - reality is a bit more nuanced.
If I develop a feature for company A, then reuse it for company B, isn't A footing the bill for B? I mean, we all do that, but I think it's worth thinking about.
Ed: that is to say the time is wasted - but might very well be able to charge a premium on the experience. In my original comment I was mostly talking about making feature x for customer a, then making feature y for customer a. Where x and y are pretty much the same.
Similarly every run of the mill business-as-usual boring-as-fuck CRUD corporate internal "app" is the same, yet they still need a lot of work, and they are still hard to "estimate".