[policy] check for duplicate txids in package

Duplicates of normal transactions would be found by looking for
conflicting inputs, but this doesn't catch identical empty transactions.
These wouldn't be valid but exiting early is good and AcceptPackage's
result sanity checks assume non-duplicate transactions.

test/functional/rpc_packages.py

@@ -212,8 +212,8 @@ class RPCPackagesTest(BitcoinTestFramework):
         coin = self.wallet.get_utxo()
 
         # tx1 and tx2 share the same inputs
-        tx1 = self.wallet.create_self_transfer(utxo_to_spend=coin)
-        tx2 = self.wallet.create_self_transfer(utxo_to_spend=coin)
+        tx1 = self.wallet.create_self_transfer(utxo_to_spend=coin, fee_rate=DEFAULT_FEE)
+        tx2 = self.wallet.create_self_transfer(utxo_to_spend=coin, fee_rate=2*DEFAULT_FEE)
 
         # Ensure tx1 and tx2 are valid by themselves
         assert node.testmempoolaccept([tx1["hex"]])[0]["allowed"]
@@ -222,8 +222,8 @@ class RPCPackagesTest(BitcoinTestFramework):
         self.log.info("Test duplicate transactions in the same package")
         testres = node.testmempoolaccept([tx1["hex"], tx1["hex"]])
         assert_equal(testres, [
-            {"txid": tx1["txid"], "wtxid": tx1["wtxid"], "package-error": "conflict-in-package"},
-            {"txid": tx1["txid"], "wtxid": tx1["wtxid"], "package-error": "conflict-in-package"}
+            {"txid": tx1["txid"], "wtxid": tx1["wtxid"], "package-error": "package-contains-duplicates"},
+            {"txid": tx1["txid"], "wtxid": tx1["wtxid"], "package-error": "package-contains-duplicates"}
         ])
 
         self.log.info("Test conflicting transactions in the same package")