I often come across merchants accepting payment in bitcoin, who require several confirmations on transactions, but where the probability of a successful double spend doesn't justify the waiting time for the user. A good example is membership payment for adult sites, where the membership can easily be withdrawn in case of a double spend, and not much is lost if a double spend is successful.
A part of the reason they do this, I think, is that they don't have a good way of knowing if a user is attempting to double spend coins sent to them.
Is there a service available that allows merchants to be notified of double spends of coins sent to them, so that they can be notified in case this happens, and take action - for example by banning the user, or just temporarily removing the deposit to an account until the transaction has 6 confirmations?
A simple service could be one where the merchant enters their deposit address at a web service, and the web service simply responds with the amount of attempted double spends, ie. if any of the inputs to the transaction that sends funds to the merchant's deposit address are attempted spent in another transaction, the request will return a number above 0.
Does such a service exist?