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