f avoid racy double insert

Author: TheBlueMatt

lightning-block-sync/src/gossip.rs

@@ -167,10 +167,18 @@ impl<S: FutureSpawner,
 			process_block!(block);
 			{
 				let mut recent_blocks = block_cache.lock().unwrap();
-				if recent_blocks.len() >= BLOCK_CACHE_SIZE {
-					recent_blocks.pop_front();
+				let mut insert = true;
+				for (height, _) in recent_blocks.iter() {
+					if *height == block_height {
+						insert = false;
+					}
+				}
+				if insert {
+					if recent_blocks.len() >= BLOCK_CACHE_SIZE {
+						recent_blocks.pop_front();
+					}
+					recent_blocks.push_back((block_height, block));
 				}
-				recent_blocks.push_back((block_height, block));
 			}
 			break 'tx_found;
 		};