With Diana's ability, you basically predict twice correct?

No, that is not correct. Diana, Lunari's ability does not have you "Predict twice."

According to her card text, her ability reads: "When a showdown begins here, you may pay [1]. If you do, [Predict], then reveal the top card of your Main Deck. If it's a spell, draw it."

Here is the breakdown of why this is not predicting twice:

1. [Predict]: This is the game action of looking at the top card of your deck and choosing to recycle it or leave it on top (and you may rearrange them if you predicted more than one card).
2. Then reveal the top card: This is a separate instruction that happens after the Predict action has been performed.
3. Drawing: You only draw a card if the card revealed by that specific "reveal" instruction is a spell.

In short, Diana's ability instructs you to Predict once, then perform a separate "reveal and potentially draw" action.