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As I understand it, in a situation where two miners solve a block at the same time, a branch is created on the blockchain which is resolved when one branch is built upon, and thus becomes longer than the other branch. The longer blockchain is then regarded as the de facto blockchain.

But what happens to the confirmed transactions in the shorter branch? If this shorter branch is now disregarded, are the transactions lost? If so, are bitcoins lost as a result? Are the transaction sent back to the miners' mempool? If so, is there a risk of double-spending?

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The transactions on the sort - or orphaned - branch are not considered confirmed. They are considered never having happened at all and are returned to the mempool, unless they were also included on the other, de facto branch.

Any bitcoins mined in an orphan block is never considered to having existed in the first place, to be able to be considered 'lost'. All the effort and electricity spent mining an orphan block is literally wasted.

There's a risk of double spend for every merchant or recipient of a transaction who didn't wait enough time to see the transaction confirmed in a block and several successive blocks. If he or she accepts a transaction in a block A, then delivers the product sold, the another transaction with the same input but other output is included in block B, then block A get orphaned, the merchant delivered the product and didn't receive the money. He or she can contact the buyer to settle this somehow, but that's the definition of a double spend.

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