My textbook says that you can use this construction "needn't have" if you want to say that something that you have done in the past wasn't necessary and you didn't know it was unnecessary, but you have already done that.
For example:
"You needn't have bought any eggs, we already have a lot in the fridge"
So I'm curious do you say that in American English? Or is there a way to say it in American English?