Why can those failed withdrawals get mined again? If the coins have been created once, how can they be created again?
This is about transactions, not newly mined coins.
One a transaction has been signed, it's valid as long as no conflicting transactions get added to the blockchain first. Once it's been broadcast to the network it could potentially be added to the blockchain at any time.
If a signed transaction is broadcast but not immediately added to the blockchain by miners, the only way to make sure that it doesn't go through later on is to send those same inputs back to yourself and make sure the second transaction does.