interop: add L2ToL2CrossDomainMessenger predeploy invariants (#469)

* chore: add missing invariants

* chore: address review comment

* chore: run fix lint

specs/interop/predeploys.md

@@ -16,6 +16,7 @@
   - [`Identifier` Getters](#identifier-getters)
 - [L2ToL2CrossDomainMessenger](#l2tol2crossdomainmessenger)
   - [`relayMessage` Invariants](#relaymessage-invariants)
+  - [`sendMessage` Invariants](#sendmessage-invariants)
   - [Message Versioning](#message-versioning)
   - [No Native Support for Cross Chain Ether Sends](#no-native-support-for-cross-chain-ether-sends)
   - [Interfaces](#interfaces)
@@ -277,6 +278,12 @@ as well as domain binding, ie the executing transaction can only be valid on a s
 
 - The `Identifier.origin` MUST be `address(L2ToL2CrossDomainMessenger)`
 - The `_destination` chain id MUST be equal to the local chain id
+- Messages MUST NOT be relayed more than once
+
+### `sendMessage` Invariants
+
+- Sent Messages MUST be uniquely identifiable
+- It must emit the `SentMessage` event
 
 ### Message Versioning