A lang item was redefined.

Erroneous code example:

```compile_fail,E0152
#![feature(lang_items)]

#[lang = "owned_box"]
struct Foo<T>(T); // error: duplicate lang item found: `owned_box`
```

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