If I topdeck a Miracle card with Kefnet, do both the miracle card and the Kefnet copy have the Miracle Cost trigger?
No. You’re misunderstanding how Miracle works.
When you draw a card with Miracle as your first card for the turn while you control God-Eternal Kefnet, you can reveal it for Miracle, Kefnet, or both. Assuming that you reveal it for both, you’ll then have two triggers trigger and go on the stack in the order of your choice. When the miracle trigger resolves, you can cast the card itself for the miracle cost if you wish. If you don’t, then the card stays in your hand. When Kefnet’s trigger resolves, you’ll copy the revealed card and be able to cast it for 2 less than its printed mana cost. You can’t pay the miracle cost for Kefnet’s trigger because the copy’s miracle ability didn’t trigger.
