Invoke CheckSpecialTx after all normal TX checks have passed (#2673)

Otherwise duplicate-keys checks for deterministic masternodes triggers
for duplicate/identical transactions.

src/validation.cpp

@@ -658,9 +658,6 @@ bool AcceptToMemoryPoolWorker(CTxMemPool& pool, CValidationState& state, const C
         return state.DoS(100, false, REJECT_INVALID, "qc-not-allowed");
     }
 
-    if (!CheckSpecialTx(tx, chainActive.Tip(), state))
-        return false;
-
     // Coinbase is only valid in a block, not as a loose transaction
     if (tx.IsCoinBase())
         return state.DoS(100, false, REJECT_INVALID, "coinbase");
@@ -868,6 +865,12 @@ bool AcceptToMemoryPoolWorker(CTxMemPool& pool, CValidationState& state, const C
             return state.DoS(0, false, REJECT_NONSTANDARD, "too-long-mempool-chain", false, errString);
         }
 
+        // check special TXs after all the other checks. If we'd do this before the other checks, we might end up
+        // DoS scoring a node for non-critical errors, e.g. duplicate keys because a TX is received that was already
+        // mined
+        if (!CheckSpecialTx(tx, chainActive.Tip(), state))
+            return false;
+
         // If we aren't going to actually accept it but just were verifying it, we are fine already
         if(fDryRun) return true;