Treat a known Message-ID as a duplicate in the mail importer

Outlook for Mac rewrites headers on every export, so a re-exported
message never hashes the same twice. Hash-only matching stored each
re-export as a variant beside the original.

README.md

@@ -21,8 +21,10 @@ filename. Unknown or invalid dates are stored under `unknown-date/`.
 The importer regenerates `index.csv` from the archive with the full SHA-256, selected normalized
 header values, and archive path. Its duplicate rules are:
 
-- An existing SHA-256 is an exact duplicate and is skipped.
-- A repeated `Message-ID` with different bytes is preserved and reported as a variant.
+- An existing SHA-256 is a duplicate and is skipped.
+- An existing `Message-ID` is a duplicate and is skipped. Outlook rewrites headers on every
+  export, so the same message rarely hashes the same twice.
+- A message without a `Message-ID` is checked by SHA-256 only.
 - A repeated subject alone is not a duplicate.
 
 Each new message is copied to a temporary file beside its final destination, SHA-256 verified, and