1 files changed, 19 insertions, 0 deletions
diff --git a/src/swarm/sync.rs b/src/swarm/sync.rs
index 8059fe2..aa6a8f9 100644
--- a/src/swarm/sync.rs
+++ b/src/swarm/sync.rs
@@ -620,6 +620,25 @@ async fn download_and_apply_delta(
             .into_iter()
             .filter(|other| other != peer),
     );
+    if let Some(peer_state) = state.get_peer_state(peer).await {
+        let doc = Arc::clone(&state.doc);
+        let from = spec.from.clone();
+        let known = tokio::task::spawn_blocking(move || {
+            let replica = doc.blocking_read();
+            peer_state
+                .blocking_lock()
+                .sync
+                .known_beyond(&replica.doc, &from)
+        })
+        .await
+        .context("Delta check task panicked")?;
+        if known > 0 {
+            // Offered before it heard what we have: it would bring those again. Our
+            // next message tells it (`fetch_delta`); it offers afresh if still needed.
+            info!(peer = %peer, known, "Offered changes partly here already; not fetched");
+            return Ok(false);
+        }
+    }
     info!(peer = %peer, to = spec.to.len(), "Fetching changes offered as a blob");
     let started = Instant::now();
     let bytes = crate::swarm::blob::fetch_delta(state, &spec, peers).await?;