Fix duplicate codec indexes (#7446)

Related to https://github.com/paritytech/polkadot-sdk/issues/7400 and
https://github.com/paritytech/polkadot-sdk/pull/7417

We need this in order to be able to update `parity-scale-codec` to the
latest version after it's released. That's because `parity-scale-codec`
added support for checking for duplicate indexes at compile time.