consensus: Explain why fCheckDuplicateInputs can not be skipped and remove it

src/consensus/tx_check.cpp

@@ -7,7 +7,7 @@
 #include <primitives/transaction.h>
 #include <consensus/validation.h>
 
-bool CheckTransaction(const CTransaction& tx, CValidationState &state, bool fCheckDuplicateInputs)
+bool CheckTransaction(const CTransaction& tx, CValidationState& state)
 {
     // Basic checks that don't depend on any context
     if (tx.vin.empty())
@@ -31,15 +31,16 @@ bool CheckTransaction(const CTransaction& tx, CValidationState &state, bool fChe
             return state.Invalid(ValidationInvalidReason::CONSENSUS, false, "bad-txns-txouttotal-toolarge");
     }
 
-    // Check for duplicate inputs - note that this check is slow so we skip it in CheckBlock
+    // Check for duplicate inputs (see CVE-2018-17144)
-    if (fCheckDuplicateInputs) {
+    // While Consensus::CheckTxInputs does check if all inputs of a tx are available, and UpdateCoins marks all inputs
+    // of a tx as spent, it does not check if the tx has duplicate inputs.
+    // Failure to run this check will result in either a crash or an inflation bug, depending on the implementation of
+    // the underlying coins database.
     std::set<COutPoint> vInOutPoints;
-        for (const auto& txin : tx.vin)
+    for (const auto& txin : tx.vin) {
-        {
         if (!vInOutPoints.insert(txin.prevout).second)
             return state.Invalid(ValidationInvalidReason::CONSENSUS, false, "bad-txns-inputs-duplicate");
     }
-    }
 
     if (tx.IsCoinBase())
     {

src/consensus/tx_check.h

@@ -15,6 +15,6 @@
 class CTransaction;
 class CValidationState;
 
-bool CheckTransaction(const CTransaction& tx, CValidationState& state, bool fCheckDuplicateInputs=true);
+bool CheckTransaction(const CTransaction& tx, CValidationState& state);
 
 #endif // BITCOIN_CONSENSUS_TX_CHECK_H

src/test/fuzz/transaction.cpp

@@ -43,12 +43,7 @@ void test_one_input(const std::vector<uint8_t>& buffer)
     }
 
     CValidationState state_with_dupe_check;
-    const bool valid_with_dupe_check = CheckTransaction(tx, state_with_dupe_check, /* fCheckDuplicateInputs= */ true);
+    (void)CheckTransaction(tx, state_with_dupe_check);
-    CValidationState state_without_dupe_check;
-    const bool valid_without_dupe_check = CheckTransaction(tx, state_without_dupe_check, /* fCheckDuplicateInputs= */ false);
-    if (valid_with_dupe_check) {
-        assert(valid_without_dupe_check);
-    }
 
     const CFeeRate dust_relay_fee{DUST_RELAY_TX_FEE};
     std::string reason;

src/validation.cpp

@@ -3301,9 +3301,8 @@ bool CheckBlock(const CBlock& block, CValidationState& state, const Consensus::P
             return state.Invalid(ValidationInvalidReason::CONSENSUS, false, "bad-cb-multiple", "more than one coinbase");
 
     // Check transactions
-    // Must check for duplicate inputs (see CVE-2018-17144)
     for (const auto& tx : block.vtx)
-        if (!CheckTransaction(*tx, state, true))
+        if (!CheckTransaction(*tx, state))
             return state.Invalid(state.GetReason(), false, state.GetRejectReason(),
                                  strprintf("Transaction check failed (tx hash %s) %s", tx->GetHash().ToString(), state.GetDebugMessage()));