A lang item was redefined.

Erroneous code example:

#![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:

#![no_std]

See also the unstable book.