“Indeed.” Many translations have the particle as causal, “for” instead of assertive, “indeed,” as the NET does, but “for” does not seem to be the heart of the meaning here. God does not want Sarah returned because Abraham is a prophet, but because she is married.
“die, yes, die.” An emphatic translation of the Hebrew, which uses the figure of speech polyptoton (Bullinger, Figures of Speech Used in the Bible), and repeats the word “die” in different tenses.
[For more on the figure polyptoton and the emphasis it brings, as well as the way it is translated in the REV, see commentary on Genesis 2:16 and commentary on Gen. 2:17.]