diff --git a/changelog.d/15075.feature b/changelog.d/15075.feature
new file mode 100644
index 000000000000..d25a7567a4fa
--- /dev/null
+++ b/changelog.d/15075.feature
@@ -0,0 +1,2 @@
+Update the error code returned when user sends a duplicate annotation.
+
diff --git a/synapse/api/errors.py b/synapse/api/errors.py
index 9235ce653638..e1737de59b51 100644
--- a/synapse/api/errors.py
+++ b/synapse/api/errors.py
@@ -108,6 +108,10 @@ class Codes(str, Enum):
 
     USER_AWAITING_APPROVAL = "ORG.MATRIX.MSC3866_USER_AWAITING_APPROVAL"
 
+    # Attempt to send a second annotation with the same event type & annotation key
+    # MSC2677
+    DUPLICATE_ANNOTATION = "M_DUPLICATE_ANNOTATION"
+
 
 class CodeMessageException(RuntimeError):
     """An exception with integer code and message string attributes.
diff --git a/synapse/handlers/message.py b/synapse/handlers/message.py
index 8f5b658d9d28..aa90d0000d40 100644
--- a/synapse/handlers/message.py
+++ b/synapse/handlers/message.py
@@ -1337,7 +1337,11 @@ async def _validate_event_relation(self, event: EventBase) -> None:
                 relation.parent_id, event.type, aggregation_key, event.sender
             )
             if already_exists:
-                raise SynapseError(400, "Can't send same reaction twice")
+                raise SynapseError(
+                    400,
+                    "Can't send same reaction twice",
+                    errcode=Codes.DUPLICATE_ANNOTATION,
+                )
 
         # Don't attempt to start a thread if the parent event is a relation.
         elif relation.rel_type == RelationTypes.THREAD: