-------------------------------------------------
  BIP: 30
  Title: Duplicate transactions
  Author: Pieter Wuille <pieter.wuille@gmail.com>
  Status: Final
  Type: Standards Track
  Created: 2012-02-22
-------------------------------------------------

[[abstract]]
Abstract
~~~~~~~~

This document gives a specification for dealing with duplicate
transactions in the block chain, in an attempt to solve certain problems
the reference implementations has with them.

[[motivation]]
Motivation
~~~~~~~~~~

So far, the Bitcoin reference implementation always assumed duplicate
transactions (transactions with the same identifier) didn't exist. This
is not true; in particular coinbases are easy to duplicate, and by
building on duplicate coinbases, duplicate normal transactions are
possible as well. Recently, an attack that exploits the reference
implementation's dealing with duplicate transactions was described and
demonstrated. It allows reverting fully-confirmed transactions to a
single confirmation, making them vulnerable to become unspendable
entirely. Another attack is possible that allows forking the block chain
for a subset of the network.

[[specification]]
Specification
~~~~~~~~~~~~~

To counter this problem, the following network rule is introduced:

* Blocks are not allowed to contain a transaction whose identifier
matches that of an earlier, not-fully-spent transaction in the same
chain.

This rule initially applied to all blocks whose timestamp is after March
15, 2012, 00:00 UTC (testnet: February 20, 2012 00:00 UTC). It was later
extended by Commit
https://github.com/bitcoin/bitcoin/commit/ab91bf39b7c11e9c86bb2043c24f0f377f1cf514[Apply
BIP30 checks to all blocks except the two historic violations.] to apply
to all blocks except the two historic blocks at heights 91842 and 91880
on the main chain that had to be grandfathered in.

[[rationale]]
Rationale
~~~~~~~~~

Whatever solution is used, the following law must be obeyed to guarantee
sane behaviour: the set of usable transactions outputs must not be
modified by adding blocks to the chain and removing them again. This
happens during a reorganisation, and the current Bitcoin reference
implementation does not obey this law in case the temporarily added
blocks contain a duplicate transaction.

There are several potential solutions to this problem:

1.  Guarantee that all coinbases are unique, making duplicate
transactions very hard to create.
2.  Remember previous remaining outputs of a given transaction
identifier, in case a new transaction with the same identifier is added.
3.  Only allow duplicate transactions in case the previous instance of
the transaction had no spendable outputs left. Removing a block from the
chain can then safely reset the removed transaction's outputs to
nothing.

The first option is probably the most complete one, as it also
guarantees transaction identifiers are unique. However, implementing it
requires several changes that need to be accepted throughout the
network. Furthermore, it does not prevent duplicate transactions based
on earlier duplicate coinbases. The second option is impossible to
implement in a forward-compatible way, as it potentially renders
currently-invalid blocks valid. In this document we choose for the third
option, because it only requires a trivial change.

Fully-spent transactions are allowed to be duplicated in order not to
hinder pruning at some point in the future. Not allowing any transaction
to be duplicated would require evidence to be kept for each transaction
ever made.

[[backward-compatibility]]
Backward compatibility
~~~~~~~~~~~~~~~~~~~~~~

The addition of this rule only makes some previously-valid blocks
invalid. This implies that if the rule is implemented by a supermajority
of miners, it is not possible to fork the block chain in a permanent way
between nodes with and without the new rule.

[[implementation]]
Implementation
~~~~~~~~~~~~~~

A patch for the reference client can be found on
https://github.com/sipa/bitcoin/tree/nooverwritetx

This BIP was implemented in Commit
https://github.com/bitcoin/bitcoin/commit/a206b0ea12eb4606b93323268fc81a4f1f952531[Do
not allow overwriting unspent transactions (BIP 30)] There have been
additional commits to refine the implementation of this BIP.

[[acknowledgements]]
Acknowledgements
~~~~~~~~~~~~~~~~

Thanks to Russell O'Connor for finding and demonstrating this problem,
and helping test the patch.
