message: [doc] package deduplication
diff: @@ -57,3 +57,18 @@ test accepts):
  - Warning: Batched fee-bumping may be unsafe for some use cases. Users and application developers
  should take caution if utilizing multi-parent packages.
  +
  +* Transactions in the package that have the same txid as another transaction already in the mempool
  + will be removed from the package prior to submission ("deduplication").
  +
  + - *Rationale*: Node operators are free to set their mempool policies however they please, nodes
  + may receive transactions in different orders, and malicious counterparties may try to take
  + advantage of policy differences to pin or delay propagation of transactions. As such, it's
  + possible for some package transaction(s) to already be in the mempool, and there is no need to
  + repeat validation for those transactions or double-count them in fees.
  +
  + - *Rationale*: We want to prevent potential censorship vectors. We should not reject entire
  + packages because we already have one of the transactions. Also, if an attacker first broadcasts
  + a competing package or transaction with a mutated witness, even though the two
  + same-txid-different-witness transactions are conflicting and cannot replace each other, the
  + honest package should still be considered for acceptance.
  