fix: identical misbehaviour headers (#3749)

I was discussing this with @aeryz, and they asked me open a fix PR

Author: aeryz

cosmwasm/ibc-union/lightclient/ethereum/src/client.rs

@@ -323,6 +323,12 @@ pub fn verify_misbehaviour<C: ChainSpec>(
         Error::MisbehaviourCannotExist(slot_1, slot_2),
     )?;
 
+    ensure(
+        misbehaviour.update_1.update_data().finalized_header
+            != misbehaviour.update_2.update_data().finalized_header,
+        Error::IdenticalMisbehaviourHeaders,
+    )?;
+
     let current_slot =
         compute_slot_at_timestamp::<C>(client_state.genesis_time, ctx.env.block.time.seconds())
             .ok_or(Error::IntegerOverflow)?;

cosmwasm/ibc-union/lightclient/ethereum/src/errors.rs

@@ -52,6 +52,9 @@ pub enum Error {
         client_state_latest_slot: u64,
         consensus_state_slot: u64,
     },
+
+    #[error("the misbehaviour headers must be different")]
+    IdenticalMisbehaviourHeaders,
 }
 
 impl From<Error> for StdError {