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The internal implementation of the unity build feature sets the HEADER_FILE_ONLY source file property to true for each source file it incorporates into a unity build. If such source files are added to more than one target, only the first target gets the source included and the other targets miss out. A modified version of the same test case as used in #19946 (closed) demonstrates the problem:

cmake_minimum_required(VERSION 3.16)
project(unitybuild)

set(CMAKE_UNITY_BUILD YES)
add_library(checker ok1.cpp ok2.cpp)
add_library(anotherTarget ok1.cpp ok2.cpp)  # This target ends up with no sources

This probably isn't a show-stopper for the CMake 3.16.0 release, but it does highlight a limitation of the current implementation which will affect some projects (e.g. those that build multiple variants of the same source file with different flags set at the target level).