test: add tests for fundrawtx and sendmany rpcs

If the serialized transaction passed to `fundrawtransaction` contains
duplicates, they will be deserialized and added to the transaction. Add
a test to ensure this behavior is not changed during the refactor.

A user can pass any number of duplicated and unrelated addresses as an
SFFO argument to `sendmany` and the RPC will not throw an error (note,
all the rest of the RPCs which take SFFO as an argument will error if
the user passes duplicates or specifies outputs not present in the
transaction). Add a test to ensure this behavior is not changed during
the refactor.

test/functional/wallet_fundrawtransaction.py

@@ -8,10 +8,13 @@
 from decimal import Decimal
 from itertools import product
 from math import ceil
+from test_framework.address import address_to_scriptpubkey
 
 from test_framework.descriptors import descsum_create
 from test_framework.messages import (
     COIN,
+    CTransaction,
+    CTxOut,
 )
 from test_framework.test_framework import BitcoinTestFramework
 from test_framework.util import (
@@ -147,6 +150,34 @@ class RawTransactionsTest(BitcoinTestFramework):
         self.test_22670()
         self.test_feerate_rounding()
         self.test_input_confs_control()
+        self.test_duplicate_outputs()
+
+    def test_duplicate_outputs(self):
+        self.log.info("Test deserializing and funding a transaction with duplicate outputs")
+        self.nodes[1].createwallet("fundtx_duplicate_outputs")
+        w = self.nodes[1].get_wallet_rpc("fundtx_duplicate_outputs")
+
+        addr = w.getnewaddress(address_type="bech32")
+        self.nodes[0].sendtoaddress(addr, 5)
+        self.generate(self.nodes[0], 1)
+
+        address = self.nodes[0].getnewaddress("bech32")
+        tx = CTransaction()
+        tx.vin = []
+        tx.vout = [CTxOut(1 * COIN, bytearray(address_to_scriptpubkey(address)))] * 2
+        tx.nLockTime = 0
+        tx_hex = tx.serialize().hex()
+        res = w.fundrawtransaction(tx_hex, add_inputs=True)
+        signed_res = w.signrawtransactionwithwallet(res["hex"])
+        txid = w.sendrawtransaction(signed_res["hex"])
+        assert self.nodes[1].getrawtransaction(txid)
+
+        self.log.info("Test SFFO with duplicate outputs")
+
+        res_sffo = w.fundrawtransaction(tx_hex, add_inputs=True, subtractFeeFromOutputs=[0,1])
+        signed_res_sffo = w.signrawtransactionwithwallet(res_sffo["hex"])
+        txid_sffo = w.sendrawtransaction(signed_res_sffo["hex"])
+        assert self.nodes[1].getrawtransaction(txid_sffo)
 
     def test_change_position(self):
         """Ensure setting changePosition in fundraw with an exact match is handled properly."""