Excellent movie. Read below for my interpretation.

Rating: 10/10 (8 May 2019)

This movie has 2 correct explanations. That's right. You can choose to believe either one.
The first explanation is that Angier was given a cloning machine by Tesla.
The second explanation is a little more complicated. According to that this isn't a sci-fi film. There's no "clone." Tesla was buying duplicate hats and cats to trick Angier into thinking it was a clone and that the machine worked so he'd keep funding it, but Angier eventually figured it out. Angier realized that he had a much better use for his wealth: hire several doubles. Six specifically. Each would have to die (unbeknownst to them) after every run of the trick lest the secret be revealed. Angier had to kill each one. It's why his run of the trick (after the practice run where he shot the first double) was limited to just 5 shows - he could only find so many doubles.
Remember: no one cares about the man in the box. So be sure he's the one you kill, or the one your rival wants to kill. It's the same as with the bird. Either you kill it or your rival does in his efforts to sabotage you.
Also, remember the lines early in: "Never lose control of your magic trick." If the machine were creating actual clones, the control would be lost (what do they decide to do, where do they show up). No, those are hired doubles. Plus, the film would be legitimately terrible to use such a bad and impossible 1950s sci-fi gimmick (cloned via simple electricity with the exact clothes and memories? Ummm. Ok).