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
```
9743432034