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Apple has double sourced custom chip designs before.

What would you say is the best example of this for being comparable to this hypothetical scenario?

They mint enough cash to hire 2 (or more!) complete back-end design teams, and given a hardware variant of Brooks Law, two fully staffed tape-outs might actually be safer than throwing all the bodies at just one.

Of course money is (relatively) no object here, which makes the rumor have any plausibility to begin with. At some point, whether it's only the chief architects, the lead engineers, etc. you've got have a common review body for monitoring progress on both designs. That doesn't sound conducive to an aggressive time to market schedule, in general.
 
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