Make claimpegin fail in the case of a double claim

Currently it will commit to the transaction without it being able to
enter the mempool. This makes the block creation code include the tx
into the block and create an invalid block.

src/wallet/rpcwallet.cpp

@@ -5317,6 +5317,15 @@ UniValue claimpegin(const JSONRPCRequest& request)
         throw JSONRPCError(RPC_DESERIALIZATION_ERROR, "TX decode failed");
     }
 
+    // To check if it's not double spending an existing pegin UTXO, we check mempool acceptance.
+    CValidationState acceptState;
+    bool accepted = ::AcceptToMemoryPool(mempool, acceptState, MakeTransactionRef(mtx), nullptr /* pfMissingInputs */,
+                            nullptr /* plTxnReplaced */, false /* bypass_limits */, maxTxFee, true /* test_accept */);
+    if (!accepted) {
+        std::string strError = strprintf("Error: The transaction was rejected! Reason given: %s", FormatStateMessage(acceptState));
+        throw JSONRPCError(RPC_WALLET_ERROR, strError);
+    }
+
     // Send it
     CValidationState state;
     mapValue_t mapValue;