Function check_sent_duplicate_parameters 

fn check_sent_duplicate_parameters(
    parameters: &[KeyValuePair],
) -> Result<(), CodecError>

The sender’s half: refuse every repeated Parameter Type but the one whose definition allows it.

Wider than check_received_duplicate_parameters on purpose — see the note above this function’s neighbours. A repeat this codec writes is a frame nothing downstream agrees on: code that scans a parameter list for a key takes whichever copy it meets first, so one frame carrying two values for one type is read two ways by two conforming implementations. That is what makes the sender’s half a MUST NOT rather than advice.