A builtin-macro was defined more than once.

Erroneous code example:

```compile_fail,E0773
#![feature(decl_macro)]
#![feature(rustc_attrs)]

#[rustc_builtin_macro]
pub macro test($item:item) {
    /* compiler built-in */
}

mod inner {
    #[rustc_builtin_macro]
    pub macro test($item:item) {
        /* compiler built-in */
    }
}
```

To fix the issue, remove the duplicate declaration:

```
#![feature(decl_macro)]
#![feature(rustc_attrs)]

#[rustc_builtin_macro]
pub macro test($item:item) {
    /* compiler built-in */
}
```

In very rare edge cases, this may happen when loading `core` or `std` twice,
once with `check` metadata and once with `build` metadata.
For more information, see [#75176].

[#75176]: https://github.com/rust-lang/rust/pull/75176#issuecomment-683234468