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Something like that, but it could be done ahead of time - receive a bitcoin address & redemption code online, send your coins then ~30mins later go to the machine, enter your redemption code and receive your BTC.

Redemption code could also be a QR code. Put it on your phone & hold it up to the machine.

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You can instant accept any bitcoin value that is lower to the investment to do the 51% attack or similar. Example: no one is going to make a 51% attack to steal only 10 bitcoin from a vending machine.

Simply put a limit, or just say to the user that any amount of bitcoin over X will need Y time to get verified. Anything lower that X amount will be instantly processed.

Doesn't it require ~10 minutes to solidify into the record to prevent double-spending without having the 51% attack?

Correct - there are various attack vectors besides the 51% attack. Ideally, a transaction shouldn't be closed until there have been 2 confirmations.

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