[bug] CMakeToolchain/CMakeDeps generates CMake warnings when dependency was already found via find_package #10132

Environment Details (include every applicable attribute)

• Operating System+version: Linux Mint 20.1 Ulyssa
• Compiler+version: gcc 11.1.0
• Conan version: 1.43.0
• Python version: 3.8.10

Steps to reproduce (Include if Applicable)

Use the CMakeToolchain and CMakeDeps generator.
Use the generated conan_toolchain.cmake file then find_package for a dependency and then find_package for a transitive dependency. E.g.

find_package(PNG 1.6 REQUIRED)
find_package(Freetype 2.10 REQUIRED)

In this example, Freetype depends on PNG.

This could possibly be avoided by omitting the find_package for PNG (in this case), but that's not a good solution in case the consumer depends on PNG (or other transitively dependent library) and can't depend on the library being included transitively.

Also, adding QUIET to the find_package calls does not suppress the warning because they are emitted directly via message(WARNING) and so cannot be suppressed with that flag.

Logs (Executed commands with output) (Include/Attach if Applicable)

CMake Warning at cmake-build-debug/zlibTargets.cmake:24 (message):
  Target name 'zlib::zlib' already exists.
Call Stack (most recent call first):
  cmake-build-debug/zlib-config.cmake:11 (include)
  [omitted path to cmake]/cmake-3.21/Modules/CMakeFindDependencyMacro.cmake:47 (find_package)
  cmake-build-debug/libpng-config.cmake:16 (find_dependency)
  [omitted path to cmake]/cmake-3.21/Modules/CMakeFindDependencyMacro.cmake:47 (find_package)
  cmake-build-debug/freetype-config.cmake:16 (find_dependency)
  CMakeLists.txt:83 (find_package)