cryptonote_protocol: prevent duplicate txs in fluff queue

1. Fix duplicate transaction #9335
2. Add test for cases where there are duplicate transaction in fluff

Co-authored-by: Boog900 <boog900@tutanota.com>

src/cryptonote_protocol/levin_notify.cpp

@@ -396,6 +396,8 @@ namespace levin
         for (auto& connection : connections)
         {
           std::sort(connection.first.begin(), connection.first.end()); // don't leak receive order
+          connection.first.erase(std::unique(connection.first.begin(), connection.first.end()),
+                                  connection.first.end());
           make_payload_send_txs(*zone_->p2p, std::move(connection.first), connection.second, zone_->pad_txs, true);
         }