[txdownload] remove unique_parents that we already have

This means we no longer return parents we already have in the
m_unique_parents result from MempoolRejectedTx.

We need to separate the loop that checks AlreadyHave parents from the
loop that adds parents as announcements, because we may do the latter
loop multiple times for different peers.

src/node/txdownloadman_impl.cpp

@@ -348,6 +348,12 @@ node::RejectedTxTodo TxDownloadManagerImpl::MempoolRejectedTx(const CTransaction
                 }
             }
             if (!fRejectedParents) {
+                // Filter parents that we already have.
+                // Exclude m_lazy_recent_rejects_reconsiderable: the missing parent may have been
+                // previously rejected for being too low feerate. This orphan might CPFP it.
+                std::erase_if(unique_parents, [&](const auto& txid){
+                    return AlreadyHaveTx(GenTxid::Txid(txid), /*include_reconsiderable=*/false);
+                });
                 const auto current_time{GetTime<std::chrono::microseconds>()};
 
                 for (const uint256& parent_txid : unique_parents) {
@@ -357,11 +363,7 @@ node::RejectedTxTodo TxDownloadManagerImpl::MempoolRejectedTx(const CTransaction
                     // Eventually we should replace this with an improved
                     // protocol for getting all unconfirmed parents.
                     const auto gtxid{GenTxid::Txid(parent_txid)};
-                    // Exclude m_lazy_recent_rejects_reconsiderable: the missing parent may have been
-                    // previously rejected for being too low feerate. This orphan might CPFP it.
-                    if (!AlreadyHaveTx(gtxid, /*include_reconsiderable=*/false)) {
-                        AddTxAnnouncement(nodeid, gtxid, current_time, /*p2p_inv=*/false);
-                    }
+                    AddTxAnnouncement(nodeid, gtxid, current_time, /*p2p_inv=*/false);
                 }
 
                 // Potentially flip add_extra_compact_tx to false if AddTx returns false because the tx was already there