diff --git a/chain/sub/incoming.go b/chain/sub/incoming.go
index 07b3343d2..d51c481d1 100644
--- a/chain/sub/incoming.go
+++ b/chain/sub/incoming.go
@@ -172,25 +172,24 @@ func fetchCids(
 	cids []cid.Cid,
 	cb func(int, blocks.Block) error,
 ) error {
-	fetchedBlocks := bserv.GetBlocks(ctx, cids)
+
+	ctx, cancel := context.WithCancel(ctx)
+	defer cancel()
 
 	cidIndex := make(map[cid.Cid]int)
 	for i, c := range cids {
 		cidIndex[c] = i
 	}
+	if len(cids) != len(cidIndex) {
+		return fmt.Errorf("duplicate CIDs in fetchCids input")
+	}
+
+	fetchedBlocks := bserv.GetBlocks(ctx, cids)
 
 	for i := 0; i < len(cids); i++ {
 		select {
 		case block, ok := <-fetchedBlocks:
 			if !ok {
-				// Closed channel, no more blocks fetched, check if we have all
-				// of the CIDs requested.
-				// FIXME: Review this check. We don't call the callback on the
-				//  last index?
-				if i == len(cids)-1 {
-					break
-				}
-
 				return fmt.Errorf("failed to fetch all messages")
 			}
 
diff --git a/chain/sub/incoming_test.go b/chain/sub/incoming_test.go
new file mode 100644
index 000000000..215439209
--- /dev/null
+++ b/chain/sub/incoming_test.go
@@ -0,0 +1,63 @@
+package sub
+
+import (
+	"context"
+	"testing"
+
+	address "github.com/filecoin-project/go-address"
+	"github.com/filecoin-project/lotus/chain/types"
+	blocks "github.com/ipfs/go-block-format"
+	"github.com/ipfs/go-cid"
+)
+
+type getter struct {
+	msgs []*types.Message
+}
+
+func (g *getter) GetBlock(ctx context.Context, c cid.Cid) (blocks.Block, error) { panic("NYI") }
+
+func (g *getter) GetBlocks(ctx context.Context, ks []cid.Cid) <-chan blocks.Block {
+	ch := make(chan blocks.Block, len(g.msgs))
+	for _, m := range g.msgs {
+		by, err := m.Serialize()
+		if err != nil {
+			panic(err)
+		}
+		b, err := blocks.NewBlockWithCid(by, m.Cid())
+		if err != nil {
+			panic(err)
+		}
+		ch <- b
+	}
+	close(ch)
+	return ch
+}
+
+func TestFetchCidsWithDedup(t *testing.T) {
+	msgs := []*types.Message{}
+	for i := 0; i < 10; i++ {
+		msgs = append(msgs, &types.Message{
+			From: address.TestAddress,
+			To:   address.TestAddress,
+
+			Nonce: uint64(i),
+		})
+	}
+	cids := []cid.Cid{}
+	for _, m := range msgs {
+		cids = append(cids, m.Cid())
+	}
+	g := &getter{msgs}
+
+	// the cids have a duplicate
+	res, err := FetchMessagesByCids(context.TODO(), g, append(cids, cids[0]))
+
+	t.Logf("err: %+v", err)
+	t.Logf("res: %+v", res)
+	if err == nil {
+		t.Errorf("there should be an error")
+	}
+	if err == nil && (res[0] == nil || res[len(res)-1] == nil) {
+		t.Fatalf("there is a nil message: first %p, last %p", res[0], res[len(res)-1])
+	}
+}
diff --git a/chain/sync.go b/chain/sync.go
index 240d1edef..c280e3a40 100644
--- a/chain/sync.go
+++ b/chain/sync.go
@@ -217,6 +217,12 @@ func (syncer *Syncer) Stop() {
 // This should be called when connecting to new peers, and additionally
 // when receiving new blocks from the network
 func (syncer *Syncer) InformNewHead(from peer.ID, fts *store.FullTipSet) bool {
+	defer func() {
+		if err := recover(); err != nil {
+			log.Errorf("panic in InformNewHead: ", err)
+		}
+	}()
+
 	ctx := context.Background()
 	if fts == nil {
 		log.Errorf("got nil tipset in InformNewHead")
@@ -1281,9 +1287,11 @@ func (syncer *Syncer) collectHeaders(ctx context.Context, incoming *types.TipSet
 
 	blockSet := []*types.TipSet{incoming}
 
+	// Parent of the new (possibly better) tipset that we need to fetch next.
 	at := incoming.Parents()
 
-	// we want to sync all the blocks until the height above the block we have
+	// we want to sync all the blocks until the height above our
+	// best tipset so far
 	untilHeight := known.Height() + 1
 
 	ss.SetHeight(blockSet[len(blockSet)-1].Height())
@@ -1377,13 +1385,17 @@ loop:
 	}
 
 	base := blockSet[len(blockSet)-1]
-	if base.Parents() == known.Parents() {
-		// common case: receiving a block thats potentially part of the same tipset as our best block
+	if base.IsChildOf(known) {
+		// common case: receiving blocks that are building on top of our best tipset
 		return blockSet, nil
 	}
 
-	if types.CidArrsEqual(base.Parents().Cids(), known.Cids()) {
-		// common case: receiving blocks that are building on top of our best tipset
+	knownParent, err := syncer.store.LoadTipSet(known.Parents())
+	if err != nil {
+		return nil, xerrors.Errorf("failed to load next local tipset: %w", err)
+	}
+	if base.IsChildOf(knownParent) {
+		// common case: receiving a block thats potentially part of the same tipset as our best block
 		return blockSet, nil
 	}