[Boost-build] Why is thread-for-install considering two identical options?

Subject: [Boost-build] Why is thread-for-install considering two identical options?
From: Kuhl, Brian (brian.kuhl_at_[hidden])
Date: 2018-09-23 03:16:02

I'm not sure why I get this new error?
All I did was move a few of the parameters I invoke b2 with into a project-config.jam as requirements.

project : requirements
          <static-only>on:<link>static
          <static-only>on,<link>shared:<build>no
          <static-only>kernel:<link>static
          <static-only>kernel,<link>shared:<build>no
          <threadapi>pthread
          <binary-format>elf
          <abi>sysv
          <threading>multi
          <target-os>vxworks

.. error in configuration checks....

Performing configuration checks
    - default address-model    : 64-bit
    - default architecture     : x86
    - symlinks supported       : yes
    - C++11 mutex              : no
    - lockfree boost::atomic_flag : yes
    - has_icu builds           : no
    - zlib                     : yes
    - bzip2                    : yes
    - lzma                     : no
    - gcc visibility           : yes
    - long double support      : yes
    - BOOST_COMP_GNUC >= 4.3.0 : no
error: No best alternative for ./thread-for-install
    next alternative: required properties: <abi>sysv <binary-format>elf <target-os>vxworks <threadapi>pthread <threading>multi
        matched
    next alternative: required properties: <abi>sysv <binary-format>elf <target-os>vxworks <threadapi>pthread <threading>multi
        matched
error: No best alternative for ./thread-for-install
    next alternative: required properties: <abi>sysv <binary-format>elf <target-os>vxworks <threadapi>pthread <threading>multi
        matched
    next alternative: required properties: <abi>sysv <binary-format>elf <target-os>vxworks <threadapi>pthread <threading>multi
        matched
Component configuration:
    - atomic                   : building
    - chrono                   : building
    - container                : not building
    - context                  : building
    - contract                 : not building
    - coroutine                : building
    - date_time                : building
    - exception                : building
    - fiber                    : not building
    - filesystem               : building
    - graph                    : building
    - graph_parallel           : not building
    - iostreams                : building
    - locale                   : not building
    - log                      : not building
    - math                     : building
    - mpi                      : not building
    - program_options          : building
    - python                   : not building
    - random                   : building
    - regex                    : building
    - serialization            : building
    - signals                  : not building
    - stacktrace               : not building