Ultimately though, as Mark Hamill noted, both Luke Skywalker and Darth Vader’s lines in that Empire Strikes Back scene were redone later on, which isn’t surprising considering that ADR is a common practice in filmmaking.
It’s also worth mentioning that the lines David Prowse said to Hamill on set that day weren’t the same ones heard in The Empire Strikes Back itself.
Vader being Luke’s father was kept secret by Hamill, George Lucas and director Irvin Kershner, with Hamill only being told moments before the scene was shot.
Instead, Prowse was told to say that Obi-Wan Kenobi was Luke’s father, so he and the rest of the cast and crew didn’t learn the truth until the movie came out.