title: Conan and Cmake: target already exists
body: <p>I use the conan_cmake_run() macro for CMake. When I add two libs with the same external dep. it results in an error message.</p>
  <p>Example:</p>
  <pre><code>conan_cmake_run(REQUIRES boost/1.74.0 BASIC_SETUP CMAKE_TARGETS)
  conan_cmake_run(REQUIRES hdf5/1.10.6  BASIC_SETUP CMAKE_TARGETS)
  </code></pre>
  <p>Boost and HDF5 will add zlib as an external dep. due to that, this results in the following error message during CMake configure.</p>
  <pre><code>CMake Error at build/conanbuildinfo_multi.cmake:152 (add_library):
    add_library cannot create imported target &quot;CONAN_PKG::zlib&quot; because another
    target with the same name already exists.
  Call Stack (most recent call first):
    build/conanbuildinfo_multi.cmake:286 (conan_define_targets)
    cmake/modules/conan.cmake:515 (conan_basic_setup)
    CMakeLists.txt:104 (conan_cmake_run)
  </code></pre>
  <p>Is there a way to explicitly not add the &quot;CONAN_PKG::zlib&quot; for boost/1.74.0 or hdf5/1.10.6?</p>
  <p>Thanks a lot in advance!</p>
  <p>Best,</p>