How many blocks are created at one point of time?

Question

How many blocks are created at one point of time? Can miners create a block?
If Yes, then how can we stop the same transaction to be entered in two different block?

Answer 1

I would rephrase your questions to: how does the Ethereum blockchain work? Currently Ethereum is a Proof of Work Blockchain.

This is a simplified Explanation of how it works: Proof of Work means that Miners try to solve a brute force puzzle, so that randomly one Miner wins and gets some Ether as a reward. The miners solve a Hash function puzzle: they have to bundle all pending transactions and generate a hash out of them that starts with a predetermined amount of zeros. They add a random value, the nounce by doing so and hash it together with the transaction. Then they look at the result and if it has the amounts of zeros they publish the block and winn the price.

So yes, miners create blocks.
The chain of all blocks together with the hash and the nounce are the blockchain. The number of zeros to solve changes automatically so that on average every 14s a block is mined, is added to the chain.

If one transaction goes into two blocks, the next random miner will eventually not accept the block and mine the previous block again, a fork happend. It has been show that as long as the majority of miners accept the published rules, the blockchain will eventually always convert to the consistent state of no double spent transaction. That is the fundamental reason why blockchain work. After a fork, the longer chain sticks to the rules and eventually the double spending chain dies out.