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Most likely because it's of strategic importance to them and they don't want to depend on a third party for a critical piece of their infrastructure.

> I do not know why microsoft dupplicates this effort.

Why do you say they are duplicating efforts?

The builds on the RedHat page are for: x86 (32-bit), x64 (64-bit x86) but any mention of aarch64, which is what Microsoft has been working on with Azul Systems, is conspicuously missing.

(aarch64 or ARM64 is the 64-bit extension of the ARM architecture.)

There are many other OpenJDK vendors: https://en.wikipedia.org/wiki/OpenJDK#OpenJDK_builds