Rustdoc allows multiple declarations of a feature

[Nemo157]

I tried this code:

#![feature(doc_cfg)]
#![feature(doc_cfg)]

fn main() {
    println!("Hello, world!");
}

I expected to see this happen: Both cargo build and cargo doc to fail because of multiple declarations of the doc_cfg feature.

Instead, this happened: cargo build failed while cargo doc succeeded:

> cargo build
   Compiling foo v0.1.0 (/tmp/tmp.NidTpZ70xE/foo)
error[E0636]: the feature `doc_cfg` has already been declared
 --> src/main.rs:2:12
  |
2 | #![feature(doc_cfg)]
  |            ^^^^^^^

error: aborting due to previous error

For more information about this error, try `rustc --explain E0636`.
error: could not compile `foo`

To learn more, run the command again with --verbose.

> cargo doc
 Documenting foo v0.1.0 (/tmp/tmp.NidTpZ70xE/foo)
    Finished dev [unoptimized + debuginfo] target(s) in 0.48s

Meta

rustc --version --verbose:

rustc 1.52.0-nightly (07194ffcd 2021-02-10)
binary: rustc
commit-hash: 07194ffcd25b0871ce560b9f702e52db27ac9f77
commit-date: 2021-02-10
host: x86_64-unknown-linux-gnu
release: 1.52.0-nightly
LLVM version: 11.0.1

[GuillaumeGomez]

Indeed, adding a fix!