compiler/rustc_error_codes/src/error_codes/E0636.md

#### Note: this error code is no longer emitted by the compiler.

The same feature is enabled multiple times with `#![feature]` attributes

Erroneous code example:

```compile_fail
#![allow(stable_features)]
#![feature(rust1)]
#![feature(rust1)] // error: the feature `rust1` has already been enabled
```