Should warn for enabling the same feature twice

[scottmcm]

I just noticed that rustc_mir is enabling in_band_lifetimes twice:

rust/src/librustc_mir/lib.rs

Line 18 in 88e0ff1

#![feature(in_band_lifetimes)]

rust/src/librustc_mir/lib.rs

Line 30 in 88e0ff1

#![feature(in_band_lifetimes)]

Would be good to have a warning to help people notice this kind of superfluous code.

[varkor]

This is fixed by #52644.