When someone attempts to double spend, it doesn't matter which happened first, any two or more transactions that reference the same UTXO should void each other out and the UTXO should be destroyed
So, I send you 25 BTC, wait for you to accept it and send me my physical item, then I spend the same UTXO to myself and both are destroyed by your new rule, giving me my money back. This is exactly the same as a regular double spend.