It is all rather confusing from a historical point of view when we did O levels and GCSEs we were awarded grades A to C for passes and D to E or U if we had failed the O levels. For the GCSEs you had two certificates if you got anything between A to C you would get a 1 in your CSE certificate and the grades went correspondingly down from D to E to U to higher numerical numbers up to 5 and U.
The numerical system now is the opposite of what it was once.
From memory, GCSEs came in around 1988 and replaced O level/CSEs; I am old enought to have been teaching then!
GCSEs brought in coursework and in Maths we had the choice of 0%, 20% or 40%; students did two or three pieces completely in class. At the time it was a breath of fresh air for students to work on different problems and, with no League tables, no-one even thought about 'giving too much help'.
I don't remember any 'double certification'; an old CSE grade 1 was considered equivalent to an O level pass.