🔒 Collision protection

[transmissions11]

TLDR: Don't allow two requests with the same execHash to be executed twice.

I used to think this was unsafe because someone could frontrun resubmitted requests and prevent it from being executed later by executing it early, I'm pretty sure the Optimism Cross Domain Messenger allows replaying messages that reverted the first time.

As long as messages can be replayed, DOS shouldn't be an issue.

[transmissions11]

This does add storage costs though, so worth evaluating if just doing an auction is ultimately more efficient.

[transmissions11]

Note to self: Important to document the implicit reliance on cross domain messages being retryable.