compiler/rustc_error_codes/src/error_codes/E0636.md

   1 A `#![feature]` attribute was declared multiple times.
   2
   3 Erroneous code example:
   4
   5 ```compile_fail,E0636
   6 #![allow(stable_features)]
   7 #![feature(rust1)]
   8 #![feature(rust1)] // error: the feature `rust1` has already been declared
   9 ```