Can I reuse a nonce to retransmit the same packet using ChaCha20-Poly1305?
I understand the "sudden death" implications of reusing a nonce with ChaCha20-poly1305, but I believe this rule doesn't apply if you are transmitting exactly the same packet.


There are two aspects to consider here: (a) confidentiality (can an adversary find out anything about the content of the plaintexts) and (b) authenticity (can an adversary forge packets).  On the confidentiality front you're unquestionably right.  A passive eavesdropper doesn't learn anything new about the plaintext from seeing the same ciphertext twice.

On the authenticity front I think it's trickier, and no verdict should be reached without careful analysis.  It's safe to say that authenticating the same message twice with the same nonce doesn't allow the adversary to easily forge any other message, so you're certainly not violating the authenticated cipher's rules.  But you do need to think about how this fits in the bigger picture—the protocol level above the cipher level.  I would ask myself, for example, whether there are scenarios where an active adversary could defeat some security property by replaying a packet they've eavesdropped on a second time, or suppressing the reception of packets and then feeding them to the recipient out of order.

Protocol design is tricky.  Authenticating individual messages doesn't automatically guarantee authenticity of a sequence of interactive session of messages.