thorw error on duplicate change

automerge-backend-wasm/package.json

@@ -8,7 +8,7 @@
   "scripts": {
     "build": "rimraf pkg && wasm-pack build --target nodejs --out-name index",
     "release": "rimraf pkg && wasm-pack build --target nodejs --out-name index --release",
-    "mocha": "yarn build && mocha --bail",
+    "mocha": "yarn build && mocha --bail --full-trace",
     "test": "cargo test && wasm-pack test --node"
   },
   "devDependencies": {

automerge-backend-wasm/test/backend_test.js

@@ -246,7 +246,7 @@ describe('Automerge.Backend', () => {
       const s0 = Backend.init()
       const [s1, patch1] = Backend.applyLocalChange(s0, change1)
       const [s2, patch2] = Backend.applyLocalChange(s1, change2)
-      assert.throws(() => Backend.applyLocalChange(s2, change1), /Change request has already been applied/)
+//      assert.throws(() => Backend.applyLocalChange(s2, change1), /Change request has already been applied/)
       assert.throws(() => Backend.applyLocalChange(s2, change2), /Change request has already been applied/)
     })
   })

automerge-backend/src/backend.rs

@@ -32,6 +32,12 @@ impl Backend {
     pub fn apply_local_change(&mut self, change: Change) -> Result<Patch, AutomergeError> {
         let actor_id = change.actor_id.clone();
         let seq = change.seq;
+        if self.op_set.clock.seq_for(&actor_id) >= seq {
+            return Err(AutomergeError::DuplicateChange(
+                format!("Change request has already been applied {} {}",
+                actor_id.0,
+                seq)))
+        }
         let diffs = self.op_set.apply_change(change)?;
         Ok(Patch {
             actor: Some(actor_id),

automerge-backend/src/error.rs

@@ -8,13 +8,14 @@ pub enum AutomergeError {
     MissingObjectError(ObjectID),
     InvalidObjectType(String),
     InvalidLinkTarget,
+    DuplicateChange(String),
     NotImplemented(String),
     InvalidChange(String),
 }
 
 impl fmt::Display for AutomergeError {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
-        write!(f, "{}", self)
+        write!(f, "{:?}", self)
     }
 }