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The "Mastering Bitcoin" book says that each block header contains a 32B long hash of the Merkle root for all the transactions it contains.

If I extract all these Merkle roots from all the blocks in the blockchain (longest branch only) and put them in a list, will that list have any duplicates?

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The merkle root is a 32 byte hash.

As the hashing operation is sha256, it is exceedingly unlikely that you will find duplicates, but it is not, strictly speaking, impossible.

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  • The list will actually have 2 duplicates, due to the coinbase duplications before BIP30.
    – Pieter Wuille
    Commented Dec 1, 2020 at 6:51
  • @PieterWuille Isn't that simply a duplicate coinbase transaction? Since block 91880 contains a non-coinbase transaction as well, the merkle root in the header will be different
    – Raghav Sood
    Commented Dec 1, 2020 at 8:15
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    Ah, I must have misremembered then! I thought they both had no other transactions.
    – Pieter Wuille
    Commented Dec 1, 2020 at 8:17

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