Refactor Exception Mapper to disallow duplicated Keys

This key is two times in this list? How does this work with mappings? What if the same keys have different values, does it throw? :thinking:

_Originally posted by @jochem-brouwer in https://github.com/ethereum/execution-spec-tests/pull/1416#discussion_r2041093251_

No file, test, or entry point is named in the issue. Locate the Exception Mapper and its key-mapping definitions, then inspect existing tests to determine how duplicate keys are currently handled; done means duplicate keys are disallowed and the behavior for conflicting values is covered.

Written by the indexing model from the issue text.