message: drakefile: Do not add backend sources twice.
diff: @@ -641,7 +641,6 @@ def configure(curl_config,
  cxx_toolkit,
  lib_cxx_config,
  )
  - sources += backend_sources
  if backend == Backends.io:
  libcoroutine_sources = drake.nodes(
  @@ -932,6 +931,7 @@ def configure(curl_config,
  if cxx_toolkit.os not in [drake.os.windows, drake.os.ios]:
  rule_install << drake.install(lib_dynamic, prefix / 'lib')
  rule_install << drake.install(lib_static, prefix / 'lib')
  + for s in [sources, backend_sources]:
  rule_install << drake.install(
  - (node for node in sources if isinstance(node, drake.cxx.Header)),
  + (node for node in s if isinstance(node, drake.cxx.Header)),
  prefix / 'include' / 'elle' / 'reactor')
  