For me and maybe for many people the clone/twin twist/secret wasn't a twist/secret at all, nor the actual point of the movie.
The real message and the real point of it for me is... well... pretty much everything else.
Cutter: Now you're looking for the secret. But you won’t find it because of course, you're not really looking. You don't really want to work it out. You want to be fooled.
Alfred Borden: The secret impresses no one. The trick you use it for is everything.
Alfred Borden: Are you watching closely?
Ackerman: We'll have to dress it up a little. Disguise it. Give them enough reason to doubt it.
Cutter: Obsession is a young man's game
Robert Angier: Would I be the man in the box or the prestige?
Robert Angier: My passion is equal to the task.
Olivia Wenscombe: It won't bring your wife back.
Robert Angier: I don't care about my wife. I care about his secret.
It's all in here. For me personally Prestige's ending isn't much different than Memento's ending or the whole movie.
In both Lenny and Angier i see the same traumatized and obsessed person reaching desperately towards everything that would be a reason for him to go on, constantly refusing to accept what has happened and go on with a normal life, instead of continuing to nurture his own mania. He doesn't want a different life let alone normal life. What he keeps doing is very masochistic and he thrives on that, he thrives on his own tragedy (as many people do) because that gives him a clear purpose. If the accident to his wife never occured he wouldnt have flipped out, yes, but he wouldn't have had the goal nor the motives for getting better at what he does (being a magician). Maybe he would've stayed being an assistant and never a real magician, he wouldn't have had a reason to learn and be agressive enough to get better.
The twin/clone trick was never a secret. We all knew it was gonna be like this and so did Angier but he forced himself to doubt it and to refuse it as a truth. Cutter told him numerous times that Borden used a double (back then it wouldnt matter if its a double or a twin because... well it doesnt really matter because its how he does his trick which is supposed to be the trick). But if he agreed to realise that Borden really used a double then this would mean he already knew Borden's secret which would've been the end of his journey and he didn't want it to end. He got addicted to his own anger and lust for revenge and that's why he went on. Revenge and Anger (i doubt the name Angier is just a coincidence) were the only things that defined him after his wife's death. It was all he had. Just like the search for his wife's murderer was all Lenny had in Memento.
And that's the relation. He wanted to keep on fooling himself so he could keep on having a purpose. Everything was obvious right from the start. The funny thing about the movie (and what's also genius about it) that the viewers also knew about the clone/twin twist but all of them (me too ofc) said... "Naah it can't be it, it's too obvious... Or at least i hope this won't be the twist, Nolan isn't THAT predictive. I mean come on...". We wanted to be fooled as well and we did. To the very end most of us (i guess) thought that this won't be the ending and then we realised it really is the ending. I could hear Nolan saying "HA!".
But that shouldn't be dissapointing because it all fits. We, the same as Angier, refused to agree that this is the truth so we would have a reason to keep on watchin the movie.
And that's how i see the movie as a whole. Another movie like Memento about the little/big obsessions that we give nutrition to, so that we have a purpose, because we are otherwise unable to find purpose in anything, because nothing would make sense. We are unable to 'pick our own destiny' and decide what we really want. Not before something tragic happens and changes our lives forever and we later on misuse and exploit this tragedy so we keep having a purpose.
EDIT: As for who else got a Tesla machine -> I'm not really sure it was Borden because I'm not sure how he would pay for it. He knew what his trick will be from the begining and he was poor then. Therefore his brother can't be a clone and he never really buy a real machine. Well he bought that electric device that added some visual spice to his trick but god knows if it's really expensive and obviously it doesn't clone people. But I'm sure he wanted Angier to see him at the Tesla exhibition so that Angier'll get suspicious and sooner or later go to Tesla (cuz that was a part of Borden's plan). Angier was predictive to Borden and Borden toyed a lot with him and his blindness to the obvious.