Fix bug where duplicate PSBT keys are accepted

As per the BIP 174 spec a PSBT key cannot be duplicated,
however the current code accepts key duplication.
The PSBT key/value entries can be duplicated when the value
is `empty()` or `IsNull()` for `CScript` or `CTxOut` respectively
and if those key/value entries are serialized before the non-empty ones.

For example, the following PSBT, included in the test vectors,
contains a duplicate field:

```
// magic
70736274ff

// global tx
//// key
0100
//// value
2a02000000000140420f000000000017a9146e91b72d5593e7d4391e2ff44e91e985c31641f08700000000
//// separator
00

// no inputs

// outputs
//// key PSBT_OUT_WITNESSSCRIPT
0101
//// value (empty script)
00
//// key PSBT_OUT_WITNESSSCRIPT (same as the above)
0101
//// value (an OP_RETURN script)
016a
//// separator
00
```

2 files changed, 28 insertions, 11 deletions

diff --git a/src/psbt.h b/src/psbt.h
index 6d77db0c6f..802a7c5ba7 100644
--- a/src/psbt.h
+++ b/src/psbt.h
@@ -126,6 +126,9 @@ struct PSBTInput
 
     template <typename Stream>
     inline void Unserialize(Stream& s) {
+        // Used for duplicate key detection
+        std::set<std::vector<unsigned char>> key_lookup;
+
         // Read loop
         bool found_sep = false;
         while(!s.empty()) {
@@ -147,7 +150,7 @@ struct PSBTInput
             switch(type) {
                 case PSBT_IN_NON_WITNESS_UTXO:
                 {
-                    if (non_witness_utxo) {
+                    if (!key_lookup.emplace(key).second) {
                         throw std::ios_base::failure("Duplicate Key, input non-witness utxo already provided");
                     } else if (key.size() != 1) {
                         throw std::ios_base::failure("Non-witness utxo key is more than one byte type");
@@ -158,7 +161,7 @@ struct PSBTInput
                     break;
                 }
                 case PSBT_IN_WITNESS_UTXO:
-                    if (!witness_utxo.IsNull()) {
+                    if (!key_lookup.emplace(key).second) {
                         throw std::ios_base::failure("Duplicate Key, input witness utxo already provided");
                     } else if (key.size() != 1) {
                         throw std::ios_base::failure("Witness utxo key is more than one byte type");
@@ -189,7 +192,7 @@ struct PSBTInput
                     break;
                 }
                 case PSBT_IN_SIGHASH:
-                    if (sighash_type > 0) {
+                    if (!key_lookup.emplace(key).second) {
                         throw std::ios_base::failure("Duplicate Key, input sighash type already provided");
                     } else if (key.size() != 1) {
                         throw std::ios_base::failure("Sighash type key is more than one byte type");
@@ -198,7 +201,7 @@ struct PSBTInput
                     break;
                 case PSBT_IN_REDEEMSCRIPT:
                 {
-                    if (!redeem_script.empty()) {
+                    if (!key_lookup.emplace(key).second) {
                         throw std::ios_base::failure("Duplicate Key, input redeemScript already provided");
                     } else if (key.size() != 1) {
                         throw std::ios_base::failure("Input redeemScript key is more than one byte type");
@@ -208,7 +211,7 @@ struct PSBTInput
                 }
                 case PSBT_IN_WITNESSSCRIPT:
                 {
-                    if (!witness_script.empty()) {
+                    if (!key_lookup.emplace(key).second) {
                         throw std::ios_base::failure("Duplicate Key, input witnessScript already provided");
                     } else if (key.size() != 1) {
                         throw std::ios_base::failure("Input witnessScript key is more than one byte type");
@@ -223,7 +226,7 @@ struct PSBTInput
                 }
                 case PSBT_IN_SCRIPTSIG:
                 {
-                    if (!final_script_sig.empty()) {
+                    if (!key_lookup.emplace(key).second) {
                         throw std::ios_base::failure("Duplicate Key, input final scriptSig already provided");
                     } else if (key.size() != 1) {
                         throw std::ios_base::failure("Final scriptSig key is more than one byte type");
@@ -233,7 +236,7 @@ struct PSBTInput
                 }
                 case PSBT_IN_SCRIPTWITNESS:
                 {
-                    if (!final_script_witness.IsNull()) {
+                    if (!key_lookup.emplace(key).second) {
                         throw std::ios_base::failure("Duplicate Key, input final scriptWitness already provided");
                     } else if (key.size() != 1) {
                         throw std::ios_base::failure("Final scriptWitness key is more than one byte type");
@@ -309,6 +312,9 @@ struct PSBTOutput
 
     template <typename Stream>
     inline void Unserialize(Stream& s) {
+        // Used for duplicate key detection
+        std::set<std::vector<unsigned char>> key_lookup;
+
         // Read loop
         bool found_sep = false;
         while(!s.empty()) {
@@ -330,7 +336,7 @@ struct PSBTOutput
             switch(type) {
                 case PSBT_OUT_REDEEMSCRIPT:
                 {
-                    if (!redeem_script.empty()) {
+                    if (!key_lookup.emplace(key).second) {
                         throw std::ios_base::failure("Duplicate Key, output redeemScript already provided");
                     } else if (key.size() != 1) {
                         throw std::ios_base::failure("Output redeemScript key is more than one byte type");
@@ -340,7 +346,7 @@ struct PSBTOutput
                 }
                 case PSBT_OUT_WITNESSSCRIPT:
                 {
-                    if (!witness_script.empty()) {
+                    if (!key_lookup.emplace(key).second) {
                         throw std::ios_base::failure("Duplicate Key, output witnessScript already provided");
                     } else if (key.size() != 1) {
                         throw std::ios_base::failure("Output witnessScript key is more than one byte type");
@@ -448,6 +454,9 @@ struct PartiallySignedTransaction
             throw std::ios_base::failure("Invalid PSBT magic bytes");
         }
 
+        // Used for duplicate key detection
+        std::set<std::vector<unsigned char>> key_lookup;
+
         // Read global data
         bool found_sep = false;
         while(!s.empty()) {
@@ -469,7 +478,7 @@ struct PartiallySignedTransaction
             switch(type) {
                 case PSBT_GLOBAL_UNSIGNED_TX:
                 {
-                    if (tx) {
+                    if (!key_lookup.emplace(key).second) {
                         throw std::ios_base::failure("Duplicate Key, unsigned tx already provided");
                     } else if (key.size() != 1) {
                         throw std::ios_base::failure("Global unsigned tx key is more than one byte type");
diff --git a/test/functional/data/rpc_psbt.json b/test/functional/data/rpc_psbt.json
index 57f2608ee9..0f6cd97fd8 100644
--- a/test/functional/data/rpc_psbt.json
+++ b/test/functional/data/rpc_psbt.json
@@ -18,7 +18,15 @@
         "cHNidP8BAHMCAAAAATAa6YblFqHsisW0vGVz0y+DtGXiOtdhZ9aLOOcwtNvbAAAAAAD/////AnR7AQAAAAAAF6kUA6oXrogrXQ1Usl1jEE5P/s57nqKHYEOZOwAAAAAXqRS5IbG6b3IuS/qDtlV6MTmYakLsg4cAAAAAAAEBHwDKmjsAAAAAFgAU0tlLZK4IWH7vyO6xh8YB6Tn5A3wCAwABAAAAAAEAFgAUYunpgv/zTdgjlhAxawkM0qO3R8sAAQAiACCHa62DLx0WgBXtQSMqnqZaGBXZ7xPA74dZ9ktbKyeKZQEBJVEhA7fOI6AcW0vwCmQlN836uzFbZoMyhnR471EwnSvVf4qHUa4A",
         "cHNidP8BAHMCAAAAATAa6YblFqHsisW0vGVz0y+DtGXiOtdhZ9aLOOcwtNvbAAAAAAD/////AnR7AQAAAAAAF6kUA6oXrogrXQ1Usl1jEE5P/s57nqKHYEOZOwAAAAAXqRS5IbG6b3IuS/qDtlV6MTmYakLsg4cAAAAAAAEBHwDKmjsAAAAAFgAU0tlLZK4IWH7vyO6xh8YB6Tn5A3wAAgAAFgAUYunpgv/zTdgjlhAxawkM0qO3R8sAAQAiACCHa62DLx0WgBXtQSMqnqZaGBXZ7xPA74dZ9ktbKyeKZQEBJVEhA7fOI6AcW0vwCmQlN836uzFbZoMyhnR471EwnSvVf4qHUa4A",
         "cHNidP8BAHMCAAAAATAa6YblFqHsisW0vGVz0y+DtGXiOtdhZ9aLOOcwtNvbAAAAAAD/////AnR7AQAAAAAAF6kUA6oXrogrXQ1Usl1jEE5P/s57nqKHYEOZOwAAAAAXqRS5IbG6b3IuS/qDtlV6MTmYakLsg4cAAAAAAAEBHwDKmjsAAAAAFgAU0tlLZK4IWH7vyO6xh8YB6Tn5A3wAAQAWABRi6emC//NN2COWEDFrCQzSo7dHywABACIAIIdrrYMvHRaAFe1BIyqeploYFdnvE8Dvh1n2S1srJ4plIQEAJVEhA7fOI6AcW0vwCmQlN836uzFbZoMyhnR471EwnSvVf4qHUa4A",
-        "cHNidP8BAHMCAAAAAbiWoY6pOQepFsEGhUPXaulX9rvye2NH+NrdlAHg+WgpAQAAAAD/////AkBLTAAAAAAAF6kUqWwXCcLM5BN2zoNqMNT5qMlIi7+HQEtMAAAAAAAXqRSVF/in2XNxAlN1OSxkyp0z+Wtg2YcAAAAAAAEBIBNssgAAAAAAF6kUamsvautR8hRlMRY6OKNTx03DK96HAQcXFgAUo8u1LWpHprjt/uENAwBpGZD0UH0BCGsCRzBEAiAONfH3DYiw67ZbylrsxCF/XXpVwyWBRgofyRbPslzvwgIgIKCsWw5sHSIPh1icNvcVLZLHWj6NA7Dk+4Os2pOnMbQBIQPGStfYHPtyhpV7zIWtn0Q4GXv5gK1zy/tnJ+cBXu4iiwABABYAFMwmJQEz+HDpBEEabxJ5PogPsqZRAAEAFgAUyCrGc3h3FYCmiIspbv2pSTKZ5jU"
+        "cHNidP8BAHMCAAAAAbiWoY6pOQepFsEGhUPXaulX9rvye2NH+NrdlAHg+WgpAQAAAAD/////AkBLTAAAAAAAF6kUqWwXCcLM5BN2zoNqMNT5qMlIi7+HQEtMAAAAAAAXqRSVF/in2XNxAlN1OSxkyp0z+Wtg2YcAAAAAAAEBIBNssgAAAAAAF6kUamsvautR8hRlMRY6OKNTx03DK96HAQcXFgAUo8u1LWpHprjt/uENAwBpGZD0UH0BCGsCRzBEAiAONfH3DYiw67ZbylrsxCF/XXpVwyWBRgofyRbPslzvwgIgIKCsWw5sHSIPh1icNvcVLZLHWj6NA7Dk+4Os2pOnMbQBIQPGStfYHPtyhpV7zIWtn0Q4GXv5gK1zy/tnJ+cBXu4iiwABABYAFMwmJQEz+HDpBEEabxJ5PogPsqZRAAEAFgAUyCrGc3h3FYCmiIspbv2pSTKZ5jU",
+        "cHNidP8BACoCAAAAAAFAQg8AAAAAABepFG6Rty1Vk+fUOR4v9E6R6YXDFkHwhwAAAAAAAQEAAQEBagA=",
+        "cHNidP8BACoCAAAAAAFAQg8AAAAAABepFG6Rty1Vk+fUOR4v9E6R6YXDFkHwhwAAAAAAAQAAAQABagA=",
+        "cHNidP8BADMBAAAAAREREREREREREREREREREREREfrK3hERERERERERERERfwAAAAD/////AAAAAAAAAQEJ//////////8AAQEJAADK/gAAAAAAAA==",
+        "cHNidP8BADMBAAAAAREREREREREREREREREREREREfrK3hERERERERERERERfwAAAAD/////AAAAAAAAAQMErd7f7gEDBAEAAAAA",
+        "cHNidP8BADMBAAAAAREREREREREREREREREREREREfrK3hERERERERERERERfwAAAAD/////AAAAAAAAAQQAAQQBagA=",
+        "cHNidP8BADMBAAAAAREREREREREREREREREREREREfrK3hERERERERERERERfwAAAAD/////AAAAAAAAAQEJAOH1BQAAAAAAAQUAAQUBUQA=",
+        "cHNidP8BADMBAAAAAREREREREREREREREREREREREfrK3hERERERERERERERfwAAAAD/////AAAAAAAAAQcAAQcBUQA=",
+        "cHNidP8BADMBAAAAAREREREREREREREREREREREREfrK3hERERERERERERERfwAAAAD/////AAAAAAAAAQEJAOH1BQAAAAAAAQgBAAEIAwEBUQA="
     ],
     "valid" : [
         "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",