Should it matter if `write_source_files` goes to the same file?

```starlark
load("@aspect_bazel_lib//lib:write_source_files.bzl", "write_source_files")
load("@bazel_skylib//rules:copy_file.bzl", "copy_file")

copy_file(
name = "foobar",
src = "//some/generated:file",
out = "foobar.json",
)

write_source_files(
name = "write_foobar",
files = {
"foobar.json": ":foobar",
},
)
```

This will complain that `foobar.json` is the same file as `foobar.json`.

Should this be an error?
They have (by definition) the same contents

I am using `copy_file` to get the file into the correct position for usage by another tool.

Start with lib:write_source_files.bzl and trace how the files mapping is checked when copy_file produces the same output path. Reproduce the shown Starlark configuration, then determine whether identical source and destination paths should be accepted; done means the behavior is decided and covered by an appropriate test.

Written by the indexing model from the issue text.