The Alphans discovering that Regina had two brains was not in the original script; it was inserted against writer Johnny Byrne's objections.
When Koenig and Carter discover the crashed Eagle and their deceased duplicates inside, Carter was piloting the Eagle and Koenig was the copilot. In the normal time line, Koenig always pilots the Eagle (right seat), and Carter is the copilot (left seat.) The name tags are shown in different places on the two versions helmets within the same frames, so it's pretty obvious that changes between the two sets of individuals was intentional and not accidental.
As Koenig, Helena, and Alan are flying above Santa Maria searching for the duplicate Alphans, right before Alan tells the Commander "There are signs of habitation below, sir", there's a deleted scene where the Eagle passes over a set of dome-like structures that could've been greenhouses (seen in the "This Episode" montage in the opening titles).
This was the first script Johnny Byrne wrote for the series (after reworking Art Wallace's "Siren Planet" into Matter of Life and Death (1975)).
Filmed April 2nd through the 19th 1974, 6th episode in production order. Barry Gray composed an original music score for the episode (with the exception of a piece taken from UFO (1970), also composed by Gray, used for Regina's hallucinatory sequence on Alpha). This episode's soundtrack was the most reused (out of all of season one's original soundtracks) in subsequent episodes.