fix interval logic

polkadot/consensus/src/service.rs

@@ -224,7 +224,11 @@ impl Service {
 				interval.map_err(|e| debug!("Timer error: {:?}", e)).for_each(move |_| {
 					if let Ok(best_block) = c.best_block_header() {
 						let hash = best_block.hash();
-						if hash == prev_best && s.live_agreement() != Some(hash) {
+						let last_agreement = s.last_agreement();
+						let can_build_upon = last_agreement
+							.map_or(true, |x| !x.live || x.parent_hash != hash);
+
+						if hash == prev_best && can_build_upon {
 							debug!("Starting consensus round after a timeout");
 							start_bft(best_block, s.clone());
 						}