Build fails if the same RMW implementation appears multiple times in the AMENT_PREFIX_PATH

[gerkey]

If you have a multi-element AMENT_PREFIX_PATH that contains the same RMW implementation twice, then you can get an error like this:

CMake Error at /Users/gerkey/ros2_ws/install/share/rmw_implementation/cmake/add_executable_for_each_rmw_implementation.cmake:66 (add_executable):
  add_executable cannot create target "intrapc__rmw_opensplice_cpp" because
  another target with the same name already exists.  The existing target is
  an executable created in source directory
  "/Users/gerkey/ros2_ws/src/ros2/demos/intra_process_comms".  See
  documentation for policy CMP0002 for more details.
Call Stack (most recent call first):
  CMakeLists.txt:21 (add_executable_for_each_rmw_implementations)

I confirmed that get_available_rmw_implementations() was returning a list with duplicates: rmw_opensplice_cpp;rmw_opensplice_cpp.

In this case, my AMENT_PREFIX_PATH was:

AMENT_PREFIX_PATH=/Users/gerkey/ros2_ws/install:/Users/gerkey/ros2_install/ros2

I had previously sourced the setup file for a binary install (the ros2_install element). I had also sourced the setup file from install tree where I was building, but that shouldn't be a problem.

Now, perhaps I'm in the wrong for getting into this configuration, but it would be good to fix, or at least get a better error message.

I'm inclined to REMOVE_DUPLICATES inside get_available_rmw_implementations(), but am open to other options.

[dirk-thomas]

Can you please double check that ament/ament_cmake#26 fixes the problem for you.

[gerkey]

Confirmed, that fixes it, thanks!