add_library cannot create imported target XXX because another target with the same name already exists.

Trying to build MuseScore using MSVC 2019 and msvc2019_64 gives:

CMake Error at C:/Qt/5.15.0/msvc2019_64/lib/cmake/Qt5Qml/Qt5Qml_QDebugMessageServiceFactory.cmake:2 (add_library):
add_library cannot create imported target
"Qt5::QDebugMessageServiceFactory" because another target with the same
name already exists.
Call Stack (most recent call first):
C:/Qt/5.15.0/msvc2019_64/lib/cmake/Qt5Qml/Qt5QmlConfigExtras.cmake:4 (include)
C:/Qt/5.15.0/msvc2019_64/lib/cmake/Qt5Qml/Qt5QmlConfig.cmake:249 (include)
build/FindQt5.cmake:40 (find_package)
CMakeLists.txt:87 (include)

It did work with beta 2.
And (still) works with msvc2017_64.