Output keys are 256 bits public keys. They should be unique.

Are there any duplicate output keys ?

What happens if there is any ? Is it a double spend ?

Here are the duplicates for block in range(1,1500000), with the count:

  • 8837 9b2e4c0281c0b02e7c53291a94d1d0cbff8883f8024f5142ee494ffbbd088071
  • 70 2a74a3c4c36d32e95633d44ba9a7b8188297b2ac91afecab826b86fabaa70916
  • 8 362da94d535b602c1ea909627da4ffb35bf01dd4689698f6393f3a5e995973b8
  • 7 0ad4bab9288ecbb1066ece2ea3fba29a013301cfa1d472da7467876635996a34
  • 6 71420acc3b5e7b3aa5405fdf06cc435c37339f8efd08c7033c4a000e0d4804b7
  • 6 ad0f4a8bb1849166d5e75c46f52b8390629f093cf100f8221f9de8f00f00f80a
  • 5 944e84a00c2e47b8ab6c1ed4e7d5325449940bd7d9947c321f2f78c6f67266cc
  • 5 52d128bc9913d5ee8b702c37609917c2357b2f587e5de5622348a3acd718e5d6
  • 4 88883ef499326c4344935c57f088758e1370a80ab757d45e0e09413485542f3b
  • 3 21242bd2219db8651fbd553339fae003d05b1c01b20424554725bd8e7834ad11
  • 3 42e782ea6c9b43d2fc101bf31db45847e2a99f91b596db1afe4851b63bdfa7d6
  • 3 b8ed916c56b3a99c9cdf22c7be7ec4e85587e5d40bc46bf6995313c288ad841e
  • 3 a6f70eae9db3e3a06000fc2cbf38d48f236d289b9ca3673e4a6edb5719e970e2
  • 2 8d820398c912ca0916e964990fd4a34ab5cda3d4f8ad992dc12fc7ee63cfa0f0
  • 2 142076b705ec912288816f0a08da20b64f9907bf99fbce74e39a85a300658191
  • 2 7e69fc5c12439bc793ec53993207ea5594ab64e2d148cb81f465c3d88f905b87
  • 2 5d397b2a3cb2788a1de2be6632fa2e389b885e59f531d11c51470ae12240f86a
  • 2 1b452b4ac6c6419e06181f8c9f0734bd5bb132d8b75b44bbcd07dd8f553acba6
  • 2 c574420256922b0783196e42c98156814e61d9ffa87c1262089be1d42e5f0d27
  • 2 b10ba13e303cbe9abf7d5d44f1d417727abcc14903a74e071abd652ce1bf76dd

As noted in this reddit thread, the first key (used 8837 times) is also the output key of the coinbase transaction in the genesis block.

Quoting smooth:

No its not a double spend. It is burning coins, so in a sense the opposite of a double spend. Possibly unintended due to a bug, but without knowing who is generating those or why it is impossible to say.

Each key can only be spent once, so out of these 8950 outputs, 8930 are unspendable.

As I've recently stumbled over this issue as well, here's a CSV file that contains all reused pubkeys and their count: reused_pubk.csv

And here also the blocks and transactions where and when they occur (Timestamp, Block, TX Hash): reused_pubk_full.csv

(Both links go to a github repo, if changes occur at some point, use this commit)

It seems that whatever bug was responsible for it has been patched at some time after 2016-10-23 (or the last client updated his wallet software back then).

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