reject duplicate Host headers in request headers

Duplicate Host headers are now rejected with a ProtocolError.
This addresses a request smuggling primitive where a consumer
downgrading HTTP/2 to HTTP/1.1 would emit two Host header lines,
causing backend divergence (CWE-444).

The _validate_host_authority_header() function now counts Host
headers and raises ProtocolError if more than one is present,
covering both the receive path (_check_host_authority_header)
and the send path (_check_sent_host_authority_header).

Author: SunandM

src/h2/utilities.py

@@ -449,6 +449,9 @@ def _validate_host_authority_header(headers: Iterable[Header]) -> Generator[Head
         if header[0] == b":authority":
             authority_header_val = header[1]
         elif header[0] == b"host":
+            if host_header_val is not None:
+                msg = "Request header block has multiple Host headers."
+                raise ProtocolError(msg)
             host_header_val = header[1]
 
         yield header