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.
Actually what you could do would be to use a single utxo and use it to send 100 of your own addresses each 1 satoshi.
Then you add more money to each address, spend as much as you want, and then cancel the original utxo. You only lose 100 satoshis, you keep all the merchandise, and you also keep all the other coins.