We will start out with the following CMake project:

cmake_minimum_required(VERSION 3.5 FATAL_ERROR)

project(recipe-06 LANGUAGES NONE)

macro(custom_include_guard)
  if(NOT DEFINED included_modules)
    set(included_modules)
  endif()

  if ("${CMAKE_CURRENT_LIST_FILE}" IN_LIST included_modules)
    message(WARNING "module ${CMAKE_CURRENT_LIST_FILE} processed more than once")
  endif()

  list(APPEND included_modules ${CMAKE_CURRENT_LIST_FILE})
endmacro()

include(cmake/custom.cmake)

message(STATUS "list of all included modules: ${included_modules}")

This code defines a custom include guard, includes a custom module (the same module as in the previous recipe), and prints the list of all included modules. For CMake 3.10 and higher, we now know from the previous recipe that there is a built-in include_guard. But instead of simply removing custom_include_guard and ${included_modules}, we will deprecate the macro and the variable with a deprecation warning, which at some point we can flip into a FATAL_ERROR to make the code stop and force the developers to switch to the built-in command.