This question already has an answer here:

I don't know how Ethereum verifies the atomicity of the state of a contract in the mining process.

Say I have a greet-counting contract.

If two people call the greet function in (almost) the same time, can both transactions be included in a single block and the counter be incremented by two?

Or only one transaction will included in a block and the other will be left in the queue for the following block? Or, the worst case, the other transaction will even be thrown away?

merged by eth May 28 '16 at 19:57

This question was merged with What happens when a smart contract gets several similar calls in the same block? because it is an exact duplicate of that question.