Allowing Duplicate Keys in BIP 390 musig() Expressions

The email highlights a specific aspect of Bitcoin Improvement Proposal (BIP) 327, which permits the inclusion of duplicate participant public keys in certain contexts. This feature is notable because it ensures that as long as all signing procedures adhere to the standards outlined in BIP 327, the process will remain valid and secure. However, the allowance of duplicate public keys introduces a complication when it comes to MuSig2 Partially Signed Bitcoin Transactions (PSBT) fields, as these are designed in a way that does not support duplicate entries.

Furthermore, the discussion brings attention to the explicit warning within BIP 327 regarding the use of deterministic nonces. The warning suggests that deterministic nonces could present issues that are separate from the matter of duplicate public keys. This implies a nuanced consideration for developers working with BIP 327, emphasizing the need to carefully navigate around the use of deterministic nonces while implementing or interacting with this proposal.