A lot of characters benefit greatly from having a key card duplicated as many times as possible. One time is good, but two times (or even more?) is fantastic.
Remove other Epiphany cards
Each Copy Card event in Chaos randomly selects 3 cards, of which you choose 1 to copy. If you want your target card to appear as one of the choices, one strategy is to remove as many alternatives as possible.
Instead of removing basic cards right away, you can instead remove other characters’ unique cards. By sacrificing their deck’s strength, you can build one deck to be especially strong.
This is easier to pull off after your characters have developed a fair amount, as survival becomes less of an issue.
Unique cards
Some characters have cards with the Unique attribute, which means they can’t be copied. Some cards gain Unique from Epiphanies. Since Unique cards will not be chosen as Copy targets, you don’t need to remove them in preparation for Copy events.
Pairing characters with these cards will make it a little bit easier to copy the card you are targeting.
The first boss is key
It’s much easier to duplicate your target cards at the first boss fight. By the time you reach the second, you just have too many cards to control it effectively.
If you haven’t duplicated your target card at the first boss fight, that might be a good time to reset your run.