duplicate operations, arguments, and leases are allowed

[bcantrill]

Through a mismerge in an Idol file, I (accidentally) discovered that the Idolatry allowed duplicate operations and arguments. On the one hand, this is not surprising, as it is Serde's default behavior (with the last key winning), but it would also be be nice to prevent this -- this is certainly an error in the Idol file, and it can be hard to debug. Unfortunately, the normal mechanism for doing this -- namely. use serde_with and then the #[serde(with = "serde_with::rust::maps_duplicate_key_is_error")] annotation -- doesn't work for IndexMap. This calls for more Serde-fu than I currently possess!

[cbiffle]

Huh, duplicate_key_is_error not working for IndexMap is disappointing. I'll look into alternatives there.