When submitting an app to the appStore, Apple will check that your have not submitted a build with the same number before. The build number also needs to be greater than your previous build numbers. Else apple will reject it.
Ensuring unique build number while using git flow and continuous integration
Olivier Destrebecq
414

This is not totally true. Actually, it’s only true for a same Version Number.

For instance, while you work on v4.2, you submit 3 builds with 1, 2 and 3 build numbers.

Then, when you work on v4.3, you can also submit builds with 1, 2, or 3, as build number.

More info in Technical Note TN2420

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