CPM usage conflicts if Boost is already included

If Boost has been added via CPM via something like:

```
add_versioned_package(
NAME Boost
VERSION 1.88.0
GITHUB_REPOSITORY "boostorg/boost"
GIT_TAG "boost-1.88.0"
)
```

then there is an error:

```
CMake Error at /path/to/cpm_cache/mp11/bf7aae5c18316ffffb3d7e62dc3f8423b7c968b1/CMakeLists.txt:9 (add_library):
add_library cannot create target "boost_mp11" because another target with
the same name already exists. The existing target is an interface library
```

Reproduce the conflict with the shown CPM add_versioned_package example and inspect the generated CMakeLists.txt where the duplicate boost_mp11 target is defined. Confirm the Boost package can already be present without a second target definition; done means the CPM setup configures without the add_library duplicate-target error.

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