ALLOW_DUPLICATE_CUSTOM_TARGETS
------------------------------

Allow duplicate custom targets to be created.

Normally CMake requires that all targets built in a project have
globally unique logical names (see policy :policy:`CMP0002`).
This is necessary to generate meaningful project file names in
:generator:`Xcode` and :ref:`Visual Studio Generators` IDE
generators.  It also allows the target names to be referenced
unambiguously.

Makefile generators are capable of supporting duplicate :command:`add_custom_target`
names.  For projects that care only about :ref:`Makefile Generators` and do
not wish to support :generator:`Xcode` or :ref:`Visual Studio Generators` IDE
generators, one may set this property to ``True``
to allow duplicate custom targets.  The property
allows multiple :command:`add_custom_target` command calls in different
directories to specify the same target name.  However, setting this
property will cause non-Makefile generators to produce an error and
refuse to generate the project.