What if 'two' blocks are mined simultaneously?

 

What happens if two miners mine the same block at the same time? Which of the blocks will be accepted and rewarded? 

 

Let’s’ say that the two miners present their valid/resolved blocks to the network. Both the miners consider the blocks as valid. Now which one to choose? And which miner to reward?

Now the decision on which block will be chosen, depends on all the miners in the bitcoin network. Both the blocks will be announced in the network and different miners will add the first block they receive to their copy of ledger, temporarily accepting that miner as the winner and allowing it to assign the reward to itself. Immediately, all the miners will stop trying to solve the current block (thinking it’s resolved) and start to work on the next block.

Now like I discussed in the ‘Orphan/Uncle blocks’, there will be two different version of the blockchain created within the short span of time. This creates two versions of chains, and some miners will consider one of the blockchains as valid while other miners will consider the other chain as valid. Now depending on factors such as the computational power of the miners working on each version /or even luck, the next succeeding block will be discovered in one of the two versions before the other. When the longest version starts spreading across the the network, the miners who had accepted the other version (short version), they have no choice but to discard the one they were working on since ‘the longest blockchain always wins in PoW’. The rejected version of the blockchain gets discarded by the network; and all the transactions included in the blocks are rejected and will be returned to mempool for miners to consider them again. In other words, transactions included in the rejected blocks will be released into the mempool so that miners will work on those returned transactions. The discarded block is the ‘orphan block’. At the same time, the miners who got the block reward from the discarded block will be removed from their balance.

Then what if the non-winning miner already spent the reward before his block reward is removed?

All transactions made within the blockchain on the orphan block are discarded, as if they had never existed. Thus, even if the miner tried to move his reward between the wallets, the end result means that these operations never existed. 

 Or is there any chances if the non-winning miners already sold the BTC/reward (that they were already rewarded) in the form of physical establishment?

This is not possible either. ‘Cos in the real world, to confirm the reward/payment to the winning miner, they have to wait for 6 accepted blocks on top the one that contains a transaction, to be considered that the block is no longer an orphan.

There could be so many orphans then? 

 
Now remember the average time that is taken to mine a block? 10 minutes time. Thus, 10 minutes is considered a perfect balance to avoid too many orphaned blocks. Otherwise, considering a few seconds taken to mint a block, there would be tons of orphan blocks and tons of versions of blockchains at the same time.