melib/thread: don't increase Thread length for duplicates

If an envelope that was already in the Threads object was inserted,
the only modification would be to erroneously increase the Thread len by
one.

melib/src/thread.rs

@@ -933,6 +933,10 @@ impl Threads {
         env_hash: EnvelopeHash,
         other_mailbox: bool,
     ) -> bool {
+        if self.hash_set.contains(&env_hash) {
+            return true;
+        }
+
         {
             let envelopes_lck = envelopes.read().unwrap();
             let message_id = envelopes_lck[&env_hash].message_id().raw();