You raised an interesting question. I found a lecture note on conditionals in an online site and it says as follows: In "informal spoken English", many people use "would have" in both the "if" clause and the result clause of conditional sentences referring to contrary to fact conditions in the past. Example: If I would have known about the quiz, I would have studied more.
New words, one handy idiom, and a 2-minute quiz — delivered to your inbox to keep your streak alive.
Paco2004Thank you paco and MrP.
Here we may read "informal spoken English" as "English spoken among people who are not well educated linguistically" and "not appropriate for academic uses" as "grammatically incorrect".
paco