Compensate for conflicting transactions?

[amiller]

If a client creates an N-way doublespend, but each node only sees one of them, then it is possible that this will not be noticed until all of them are committed.

The simplest thing to do is just reject all but the first transaction. That is, the transaction will be "committed" in the blockchain, but discarded after processing with no effect. However, this introduces a DoS vector because double-spends use up space, but are cheap. This could be related to #8 on variable size transactions.

One approach may be to require parties to reserve some "double spend insurance", which is collateral that can be taken in compensation in the case of a double-spend.

Unfortunately requiring this much collateral could require a lot of money. Is there a tradeoff here?