Disallow duplicated enum variant indices

[nazar-pc]

Currently it is possible to do this:

#[derive(Encode, Decode)]
enum E {
    #[codec(index = 0)]
    A,
    #[codec(index = 0)]
    B,
}

Logically it seems like an error and should not compile at all. Admittedly this is a minor issue.

[gui1117]

related https://github.com/paritytech/parity-scale-codec/issues/173