Overview

A normal Bitcoin transaction uses at least one output from a previous transaction by referencing its transaction ID (TXID). These unspent outputs can only be spent once, and if they could be spent twice, you could perform a double spend, making Bitcoin worthless. However, there are exactly two sets of completely identical transactions in Bitcoin. This is possible because coinbase transactions have no transaction inputs but instead have newly generated coins. Therefore, two different coinbase transactions could potentially send the same amount to the same address and be constructed in exactly the same way, making them identical. Since these transactions are identical, their TXIDs also match, as the TXID is a hash digest of the transaction data. The only other way TXIDs could be duplicated is through a hash collision, which is considered highly unlikely and impossible for cryptographically secure hash functions. Hash collisions like SHA256 have never occurred in Bitcoin or anywhere else.

These two sets of duplicate transactions occurred within a close timeframe, between 08:37 UTC on November 14, 2010, and 00:38 UTC on November 15, 2010, spanning approximately 16 hours. The first set of duplicate transactions was sandwiched between the second set. We classify d5d2….8599 as the first duplicate transaction, as it first became a copy, although strangely, it first appeared on the blockchain after another duplicate transaction e3bf….b468.

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Since BIP34 was adopted, in November 2015, Bitcoin developer Alex Morcos added a pull request to the Bitcoin core software repository, which meant that nodes would stop performing BIP30 checks. After all, since BIP34 had corrected this issue, such an expensive check was no longer necessary. Although unknown at the time, this was technically a hard Fork for some very rare blocks in the future. Now it seems that the potential hard Fork is not important because almost no one is running node software from before November 2015. At forkmonitor.info, we are running Bitcoin Core 0.10.3 released in October 2015. Therefore, this was a rule before the hard Fork, and clients were still performing expensive BIP30 checks.

Block ,983,702 Issue

It turned out that there were some coinbase transactions in blocks before BIP34 activation, where the first byte of scriptSigs happened to match the block height valid in the future. Therefore, while BIP34 did fix this issue in almost all cases, it was not a 100% fix. In 2018, Bitcoin developer John Newbery printed out the complete list of these potentially duplicate transactions, as shown in the table below.

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