for seraphine encore, can I choose 2 of the same effects?

Radiance rules aren't published yet. This uses the printed card and current rules only, and it isn't a ruling on the new mechanics.

I don't have a "Seraphine, Encore" card (or any "Encore" card) in the card database, and "Encore" isn't a keyword in the current Core Rules, so the part that actually decides your question — the card's own wording — isn't in anything I can read. Whether you can name the same effect twice depends entirely on how that card is written (something like "choose two" vs. an explicit "that haven't been chosen yet" restriction) and on what Encore is defined to do, neither of which is printed in the material I have.

The closest printed analog is Repeat: choices for the additional execution don't have to match the initial ones, so you can pick the same mode twice unless the card says otherwise (Rule 820.2.a). That's my read based on the Repeat mechanic, and no official FAQ exists for this specific interaction — if Encore is a new mechanic, its exact definition is unknown.