As far as I can tell it is not possible to use this CMake module in projects with hierarchical CMakeLists.txt and/or multiple CMake targets.

Assuming project with an exectuable and library target it would be nice to be able to use the CMake module as follows.

project(my_app)

list(APPEND CMAKE_MODULE_PATH ${PROJECT_SOURCE_DIR}/clangformat-cmake/cmake)
include(ClangFormat)

add_library(my_lib my_lib.cpp)
target_clangformat_setup(my_lib)

add_executable(my_app my_app.cpp)
target_clangformat_setup(my_app)
target_link_libraries(my_app my_lib)

However this will currently fail because each call to target_clangformat_setup() results in a call to add_custom_target() with the same target name (${PROJECT_NAME}_clangformat).

A possible workaround would be to use clangformat_setup() directly. However, this would require to list all source files twice and does not work with hierarchical CMakeLists.txt setups: assuming you have a separate CMakeLists.txt for the executable and the library from the above example.

CMakeLists.txt for executable

project(my_app)

list(APPEND CMAKE_MODULE_PATH ${PROJECT_SOURCE_DIR}/clangformat-cmake/cmake)
include(ClangFormat)

add_subdirectory(lib)

add_executable(my_app my_app.cpp)
clangformat_setup(my_app.cpp)
target_link_libraries(my_app my_lib)

CMakeLists.txt for library

add_library(my_lib my_lib.cpp)
clangformat_setup(my_lib.cpp)

This will also fail because clangformat_setup() is called twice in the same project.

edit: I have just discovered your example repository that demonstrates usage of the CMake module. In the example, the problem is avoided by having separate project() command in the main and test CMakeLists.txt. Is this a CMake pattern when using hierarchical CMakeLists.txt? However, this will still fail if multiple targets are defined in the same CMakeLists.txt file.