diff --git a/crates/zx_document_core/src/events_v2.rs b/crates/zx_document_core/src/events_v2.rs
index ce8a633..f3f5883 100644
--- a/crates/zx_document_core/src/events_v2.rs
+++ b/crates/zx_document_core/src/events_v2.rs
@@ -196,6 +196,9 @@ pub fn mark_events_failed_v2(event_ids: &[String]) -> u32 {
             let mut count = 0;
             for item in buf.iter_mut() {
                 if event_ids.contains(&item.event.event_id) {
+                    if item.state == BufferedEventState::Failed {
+                        continue; // already Failed, skip to avoid double-counting retry
+                    }
                     item.state = BufferedEventState::Failed;
                     item.retry_count += 1;
                     count += 1;
@@ -657,6 +660,22 @@ mod tests {
         teardown_session(&sid);
     }
 
+    #[test]
+    fn test_mark_failed_idempotent() {
+        let _guard = TEST_LOCK.lock().unwrap();
+        clear_all_events_v2();
+        let sid = setup_session("mat_idem");
+        let e1 = push_heartbeat_v2(&sid, "mat_idem", 15, None, 1000).unwrap();
+        export_pending_events_v2(100, 2000);
+        // Mark twice — second should be a no-op
+        let first = mark_events_failed_v2(&[e1.event_id.clone()]);
+        assert_eq!(first, 1);
+        let second = mark_events_failed_v2(&[e1.event_id.clone()]);
+        assert_eq!(second, 0, "duplicate mark_failed should return 0, not double-count");
+        ack_events_v2(&[e1.event_id.clone()]);
+        teardown_session(&sid);
+    }
+
     #[test]
     fn test_delta_semantics_per_event_type() {
         let _guard = lock_test();