Contracts receiving multiple identical transactions

Question

Assume a smart contract is used to add potatoes. Every time a person gets a new potato they send a transaction to the smart contract which adds 1 to the total potato count.

Let's assume, within a single block, a person accidentally sends two transactions to add to the potato count when they only meant to send one. Considering they are being processed in the same block, is there any way to ensure that only one adds to the global potato count?

How can you tell miners to only include one transaction of a specific type per address per block?

Answer 1

An obvious way is to use a mapping to track which accounts have sent a transaction:

contract C {
  mapping (address => bool) sentTx;
  uint public globalCount;

  function potato() {
    if (!sentTx[msg.sender]) {
      globalCount++;
      sentTx[msg.sender] = true;
    }
  }
}

This works because contract storage is updated after every transaction.

How can you tell miners to only include one transaction of a specific type per address per block?

Miners are basically in full control (subject to the mining software they are running) and decide what transactions to include in a block and in what order. The protocol imposes few rules on miners: if a transaction is valid, and a block is valid, miners can do what they want: miners don't have to exclude or include any particular transactions.