Seemingly duplicate labels

Description of the problem / feature request:

Basically it is good that Bazel forbids duplicate labels. But it should not make false-positives. For example:

cc_binary(
    name = "hello",
    srcs = select({ "//conditions:default" : [], "//:x86_32_windows" : ["sources/hello.cpp"] })
        + [ "sources/common.cpp" ]
        + select({ "//conditions:default" : [], "//:x64_windows" : ["sources/hello.cpp"] })
)

This target looks good, but in reality it won’t compile:

ERROR: C:/users/chebotarev_v/documents/src/bazel-issues/seemingly-duplicate-labels/BUILD:17:1: Label '//:sources/hello.cpp' is duplicated in the 'srcs' attribute of rule 'hello'

Feature requests: what underlying problem are you trying to solve with this feature?

Label lists can be combined with (+) and come from different points of BUILD files, so these false-positives can be extremely hard to workaround in a project. I believe that should be fixed.

Bugs: what’s the simplest, easiest way to reproduce this bug? Please provide a minimal example if possible.

Please find an example here: https://github.com/excitoon/bazel-issues/tree/master/seemingly-duplicate-labels

What operating system are you running Bazel on?

Windows 10 x64

What’s the output of bazel info release?

Build label: 0.14.0
Build target: bazel-out/x64_windows-opt/bin/src/main/java/com/google/devtools/build/lib/bazel/BazelServer_deploy.jar
Build time: Fri Jun 1 13:06:29 2018 (1527858389)
Build timestamp: 1527858389
Build timestamp as int: 1527858389

[cushon]

Consider using collections.uniq from https://github.com/bazelbuild/bazel-skylib to deduplicate the srcs of that target, e.g.:

Add this to your WORKSPACE:

git_repository(
    name = "bazel_skylib",
    remote = "https://github.com/bazelbuild/bazel-skylib.git",
    tag = "0.1.0",  # change this to use a different release
)

And then use:

load("@bazel_skylib//:lib.bzl", "collections")

cc_binary(
    name = "hello",
    srcs = collections.uniq(select({ "//conditions:default" : [], "//:x86_32_windows" : ["sources/hello.cpp"] })
        + [ "sources/common.cpp" ]
        + select({ "//conditions:default" : [], "//:x64_windows" : ["sources/hello.cpp"] }))
)

[FaBrand]

@gregestren Just came across the same issue after a bit of searching. I guess it would make sense to extend the docs with this information.

I created https://github.com/bazelbuild/bazel/pull/15714 to make this behavior easier to detect