Forbidding duplicate certificates

The implementation as of Conway forbids duplicate certificates in the same transaction. We don't have to add this restriction right away since the implementation is allowed to be more restrictive than the spec, but if we ever do negative conformance tests we may have to implement it anyway.

The issue names no files, tests, or entry points, so start by locating the Conway transaction certificate validation and the relevant conformance tests. Confirm how duplicate certificates are represented and whether negative conformance coverage is required; done means the specification and tests enforce the intended duplicate-certificate behavior.

Written by the indexing model from the issue text.