There is something called "Chaos Energy" which is part of what is needed to transform into Super, the Chaos Emeralds channel Chaos Energy to make the transmission, but if an individual is born or exposed/injected with Chaos Energy, they will not need the Chaos Emeralds to transform

Manic was not born with Chaos Energy, but when he became the God of the Multiverse he was exposed to a lot of Chaos Energy, causing some of it to join his body, giving him many abilities that a normal hedgehog doesn't have, one of those being having different transformations, what for many is a little increase in energy, for Manic it is the Super

So what would happen if Manic used the Chaos Emeralds while already in Super? Well, it would be something similar to Super Sonic 2 from Sonic Frontiers, but something more powerful, but it's not as strong as Hyper