The answer from Commsec is that both orders would be executed and twice the shares I own would be sold.
Other than it is an illegal practice so I would have to purchase more shares to fill the void from overselling they could not tell me what the penalty was if I refused or was unable to purchase more.
They referred me to the ASX which dishes out the law for naked short selling.
I have increased my spread so that the chances are slim but with the R word being mentioned so frequently anything is possible.