A lang item was redefined.
Erroneous code example:
```compile_fail,E0152
#![feature(lang_items)]
#[lang = "arc"]
struct Foo; // error: duplicate lang item found: `arc`
```
Lang items are already implemented in the standard library. Unless you are
writing a free-standing application (e.g., a kernel), you do not need to provide
them yourself.
You can build a free-standing crate by adding `#![no_std]` to the crate
attributes:
```ignore (only-for-syntax-highlight)
#![no_std]
```
See also the [unstable book][1].
[1]: https://doc.rust-lang.org/unstable-book/language-features/lang-items.html#writing-an-executable-without-stdlib