Label case insensitivity causes duplicate outputs

When Bazel builds 2 case insensitive `Label` strings, it will throw various combinations of errors or exceptions and fail.

Additionally, the duplicated outputs for multiple actions won't fail the validation check.

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

Build bazel targets from the command line using a lowercased version of the label

```
bazel build //A/B:C //a/b:c
```
_tested with native C++ compilation rules_

### What's the output of `bazel info release`?
```
$ tools/bazelwrapper info release
release 0.18.0
```

Reproduce the issue from the command line with `bazel build //A/B:C //a/b:c` using native C++ compilation rules, and record the errors or exceptions. Trace how case-insensitive `Label` strings reach multiple actions and the output validation check. Done means the command no longer fails inconsistently and duplicate outputs are detected by validation.

Written by the indexing model from the issue text.