Selling identical covers

I have a 5/0/2 Loki with two additional black covers that are about to expire. I know that somewhere out there is a thread that said that if you rostered them as a dupe then leveled it up to a certain point before selling it you maximize the ISO return, but for the life of me I can’t find that thread. Can someone help?

Just roster him as a dupe, add extra covers and level him to max then sell him.

https://forums.505go.com/discussion/48270

Thanks for a useful answer.