If we can provide deterministic way to order double-spends then we can include both and ignore the younger one. It's not related to Iota though, just an idea.
That puts undue stress on others. For instance, it no longer suffices for a merchant to see a payment confirmed by tons of PoW from followup tx.
She additionally needs to go all the way back in history to check for potential double-spends that change the status of the recent payment to "please ignore".