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How it's possible to double spend a transaction if the transaction outputs are already spent? and why these types of attacks are not preventable?

is it about blockchain sync time between nodes?

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Double spending is not possible because there are many miners checking the data independently and then verifying the result against each other. The first 'spend' that gets put into the blockchain and verified is the only one that can count - if another one comes along, it is ignored.

A simple, and elegant, way to keep double spending impossible.

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  • I'm talking about double the same transaction.
    – Adam
    Jan 20 '18 at 14:06
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    I am, too. I used the same term you used in the question (and your tag), so I'm a bit confused by your comment. Can you ask your question in a different way, or help me understand what you are looking for? Perhaps various other posts (under the 'Related' section on the right) will help - I found bitcoin.stackexchange.com/questions/50375/… explains how you CAN do it, though the end result is the same as my answer - only one transaction actually has a high enough count to verify it is valid.
    – Apps-n-Add-Ons
    Jan 20 '18 at 14:16

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