diff --git a/exchange/bitswap/bitswap.go b/exchange/bitswap/bitswap.go
index 5e00a5888a2..a785b15dc4c 100644
--- a/exchange/bitswap/bitswap.go
+++ b/exchange/bitswap/bitswap.go
@@ -78,9 +78,8 @@ func (bs *bitswap) Block(parent context.Context, k u.Key) (*blocks.Block, error)
 	go func() {
 		message := bsmsg.New()
 		for _, wanted := range bs.wantlist.Keys() {
-			message.AppendWanted(wanted)
+			message.AddWanted(wanted)
 		}
-		message.AppendWanted(k)
 		for peerToQuery := range peersToQuery {
 			log.Debugf("bitswap got peersToQuery: %s", peerToQuery)
 			go func(p peer.Peer) {
@@ -168,7 +167,7 @@ func (bs *bitswap) ReceiveMessage(ctx context.Context, p peer.Peer, incoming bsm
 
 	message := bsmsg.New()
 	for _, wanted := range bs.wantlist.Keys() {
-		message.AppendWanted(wanted)
+		message.AddWanted(wanted)
 	}
 	for _, key := range incoming.Wantlist() {
 		// TODO: might be better to check if we have the block before checking
@@ -177,7 +176,7 @@ func (bs *bitswap) ReceiveMessage(ctx context.Context, p peer.Peer, incoming bsm
 			if block, errBlockNotFound := bs.blockstore.Get(key); errBlockNotFound != nil {
 				continue
 			} else {
-				message.AppendBlock(*block)
+				message.AddBlock(*block)
 			}
 		}
 	}
@@ -207,9 +206,9 @@ func (bs *bitswap) sendToPeersThatWant(ctx context.Context, block blocks.Block)
 			log.Debugf("%v wants %v", p, block.Key())
 			if bs.strategy.ShouldSendBlockToPeer(block.Key(), p) {
 				message := bsmsg.New()
-				message.AppendBlock(block)
+				message.AddBlock(block)
 				for _, wanted := range bs.wantlist.Keys() {
-					message.AppendWanted(wanted)
+					message.AddWanted(wanted)
 				}
 				go bs.send(ctx, p, message)
 			}
diff --git a/exchange/bitswap/message/message.go b/exchange/bitswap/message/message.go
index b7216b02468..e0aea227d40 100644
--- a/exchange/bitswap/message/message.go
+++ b/exchange/bitswap/message/message.go
@@ -14,10 +14,25 @@ import (
 // TODO move bs/msg/internal/pb to bs/internal/pb and rename pb package to bitswap_pb
 
 type BitSwapMessage interface {
+	// Wantlist returns a slice of unique keys that represent data wanted by
+	// the sender.
 	Wantlist() []u.Key
+
+	// Blocks returns a slice of unique blocks
 	Blocks() []blocks.Block
-	AppendWanted(k u.Key)
-	AppendBlock(b blocks.Block)
+
+	// AddWanted adds the key to the Wantlist.
+	//
+	// Insertion order determines priority. That is, earlier insertions are
+	// deemed higher priority than keys inserted later.
+	//
+	// t = 0, msg.AddWanted(A)
+	// t = 1, msg.AddWanted(B)
+	//
+	// implies Priority(A) > Priority(B)
+	AddWanted(u.Key)
+
+	AddBlock(blocks.Block)
 	Exportable
 }
 
@@ -26,44 +41,55 @@ type Exportable interface {
 	ToNet(p peer.Peer) (nm.NetMessage, error)
 }
 
-// message wraps a proto message for convenience
-type message struct {
-	wantlist []u.Key
-	blocks   []blocks.Block
+type impl struct {
+	existsInWantlist map[u.Key]struct{}     // map to detect duplicates
+	wantlist         []u.Key                // slice to preserve ordering
+	blocks           map[u.Key]blocks.Block // map to detect duplicates
 }
 
-func New() *message {
-	return new(message)
+func New() BitSwapMessage {
+	return &impl{
+		blocks:           make(map[u.Key]blocks.Block),
+		existsInWantlist: make(map[u.Key]struct{}),
+		wantlist:         make([]u.Key, 0),
+	}
 }
 
 func newMessageFromProto(pbm pb.Message) BitSwapMessage {
 	m := New()
 	for _, s := range pbm.GetWantlist() {
-		m.AppendWanted(u.Key(s))
+		m.AddWanted(u.Key(s))
 	}
 	for _, d := range pbm.GetBlocks() {
 		b := blocks.NewBlock(d)
-		m.AppendBlock(*b)
+		m.AddBlock(*b)
 	}
 	return m
 }
 
-// TODO(brian): convert these into keys
-func (m *message) Wantlist() []u.Key {
+func (m *impl) Wantlist() []u.Key {
 	return m.wantlist
 }
 
-// TODO(brian): convert these into blocks
-func (m *message) Blocks() []blocks.Block {
-	return m.blocks
+func (m *impl) Blocks() []blocks.Block {
+	bs := make([]blocks.Block, 0)
+	for _, block := range m.blocks {
+		bs = append(bs, block)
+	}
+	return bs
 }
 
-func (m *message) AppendWanted(k u.Key) {
+func (m *impl) AddWanted(k u.Key) {
+	_, exists := m.existsInWantlist[k]
+	if exists {
+		return
+	}
+	m.existsInWantlist[k] = struct{}{}
 	m.wantlist = append(m.wantlist, k)
 }
 
-func (m *message) AppendBlock(b blocks.Block) {
-	m.blocks = append(m.blocks, b)
+func (m *impl) AddBlock(b blocks.Block) {
+	m.blocks[b.Key()] = b
 }
 
 func FromNet(nmsg netmsg.NetMessage) (BitSwapMessage, error) {
@@ -75,7 +101,7 @@ func FromNet(nmsg netmsg.NetMessage) (BitSwapMessage, error) {
 	return m, nil
 }
 
-func (m *message) ToProto() *pb.Message {
+func (m *impl) ToProto() *pb.Message {
 	pb := new(pb.Message)
 	for _, k := range m.Wantlist() {
 		pb.Wantlist = append(pb.Wantlist, string(k))
@@ -86,6 +112,6 @@ func (m *message) ToProto() *pb.Message {
 	return pb
 }
 
-func (m *message) ToNet(p peer.Peer) (nm.NetMessage, error) {
+func (m *impl) ToNet(p peer.Peer) (nm.NetMessage, error) {
 	return nm.FromObject(p, m.ToProto())
 }
diff --git a/exchange/bitswap/message/message_test.go b/exchange/bitswap/message/message_test.go
index 932c14e9bdb..9c69136cd7c 100644
--- a/exchange/bitswap/message/message_test.go
+++ b/exchange/bitswap/message/message_test.go
@@ -13,7 +13,7 @@ import (
 func TestAppendWanted(t *testing.T) {
 	const str = "foo"
 	m := New()
-	m.AppendWanted(u.Key(str))
+	m.AddWanted(u.Key(str))
 
 	if !contains(m.ToProto().GetWantlist(), str) {
 		t.Fail()
@@ -42,7 +42,7 @@ func TestAppendBlock(t *testing.T) {
 	m := New()
 	for _, str := range strs {
 		block := blocks.NewBlock([]byte(str))
-		m.AppendBlock(*block)
+		m.AddBlock(*block)
 	}
 
 	// assert strings are in proto message
@@ -58,7 +58,7 @@ func TestWantlist(t *testing.T) {
 	keystrs := []string{"foo", "bar", "baz", "bat"}
 	m := New()
 	for _, s := range keystrs {
-		m.AppendWanted(u.Key(s))
+		m.AddWanted(u.Key(s))
 	}
 	exported := m.Wantlist()
 
@@ -81,7 +81,7 @@ func TestCopyProtoByValue(t *testing.T) {
 	const str = "foo"
 	m := New()
 	protoBeforeAppend := m.ToProto()
-	m.AppendWanted(u.Key(str))
+	m.AddWanted(u.Key(str))
 	if contains(protoBeforeAppend.GetWantlist(), str) {
 		t.Fail()
 	}
@@ -101,11 +101,11 @@ func TestToNetMethodSetsPeer(t *testing.T) {
 
 func TestToNetFromNetPreservesWantList(t *testing.T) {
 	original := New()
-	original.AppendWanted(u.Key("M"))
-	original.AppendWanted(u.Key("B"))
-	original.AppendWanted(u.Key("D"))
-	original.AppendWanted(u.Key("T"))
-	original.AppendWanted(u.Key("F"))
+	original.AddWanted(u.Key("M"))
+	original.AddWanted(u.Key("B"))
+	original.AddWanted(u.Key("D"))
+	original.AddWanted(u.Key("T"))
+	original.AddWanted(u.Key("F"))
 
 	p := peer.WithIDString("X")
 	netmsg, err := original.ToNet(p)
@@ -133,10 +133,10 @@ func TestToNetFromNetPreservesWantList(t *testing.T) {
 func TestToAndFromNetMessage(t *testing.T) {
 
 	original := New()
-	original.AppendBlock(*blocks.NewBlock([]byte("W")))
-	original.AppendBlock(*blocks.NewBlock([]byte("E")))
-	original.AppendBlock(*blocks.NewBlock([]byte("F")))
-	original.AppendBlock(*blocks.NewBlock([]byte("M")))
+	original.AddBlock(*blocks.NewBlock([]byte("W")))
+	original.AddBlock(*blocks.NewBlock([]byte("E")))
+	original.AddBlock(*blocks.NewBlock([]byte("F")))
+	original.AddBlock(*blocks.NewBlock([]byte("M")))
 
 	p := peer.WithIDString("X")
 	netmsg, err := original.ToNet(p)
@@ -169,3 +169,20 @@ func contains(s []string, x string) bool {
 	}
 	return false
 }
+
+func TestDuplicates(t *testing.T) {
+	b := blocks.NewBlock([]byte("foo"))
+	msg := New()
+
+	msg.AddWanted(b.Key())
+	msg.AddWanted(b.Key())
+	if len(msg.Wantlist()) != 1 {
+		t.Fatal("Duplicate in BitSwapMessage")
+	}
+
+	msg.AddBlock(*b)
+	msg.AddBlock(*b)
+	if len(msg.Blocks()) != 1 {
+		t.Fatal("Duplicate in BitSwapMessage")
+	}
+}
diff --git a/exchange/bitswap/strategy/strategy_test.go b/exchange/bitswap/strategy/strategy_test.go
index e3ffc05ea81..ef93d98275e 100644
--- a/exchange/bitswap/strategy/strategy_test.go
+++ b/exchange/bitswap/strategy/strategy_test.go
@@ -30,7 +30,7 @@ func TestConsistentAccounting(t *testing.T) {
 
 		m := message.New()
 		content := []string{"this", "is", "message", "i"}
-		m.AppendBlock(*blocks.NewBlock([]byte(strings.Join(content, " "))))
+		m.AddBlock(*blocks.NewBlock([]byte(strings.Join(content, " "))))
 
 		sender.MessageSent(receiver.Peer, m)
 		receiver.MessageReceived(sender.Peer, m)
@@ -60,7 +60,7 @@ func TestBlockRecordedAsWantedAfterMessageReceived(t *testing.T) {
 	block := blocks.NewBlock([]byte("data wanted by beggar"))
 
 	messageFromBeggarToChooser := message.New()
-	messageFromBeggarToChooser.AppendWanted(block.Key())
+	messageFromBeggarToChooser.AddWanted(block.Key())
 
 	chooser.MessageReceived(beggar.Peer, messageFromBeggarToChooser)
 	// for this test, doesn't matter if you record that beggar sent
diff --git a/exchange/bitswap/testnet/network_test.go b/exchange/bitswap/testnet/network_test.go
index c2cc28f8d02..3930c2a8cb4 100644
--- a/exchange/bitswap/testnet/network_test.go
+++ b/exchange/bitswap/testnet/network_test.go
@@ -33,7 +33,7 @@ func TestSendRequestToCooperativePeer(t *testing.T) {
 		// TODO test contents of incoming message
 
 		m := bsmsg.New()
-		m.AppendBlock(*blocks.NewBlock([]byte(expectedStr)))
+		m.AddBlock(*blocks.NewBlock([]byte(expectedStr)))
 
 		return from, m
 	}))
@@ -41,7 +41,7 @@ func TestSendRequestToCooperativePeer(t *testing.T) {
 	t.Log("Build a message and send a synchronous request to recipient")
 
 	message := bsmsg.New()
-	message.AppendBlock(*blocks.NewBlock([]byte("data")))
+	message.AddBlock(*blocks.NewBlock([]byte("data")))
 	response, err := initiator.SendRequest(
 		context.Background(), peer.WithID(idOfRecipient), message)
 	if err != nil {
@@ -77,7 +77,7 @@ func TestSendMessageAsyncButWaitForResponse(t *testing.T) {
 		peer.Peer, bsmsg.BitSwapMessage) {
 
 		msgToWaiter := bsmsg.New()
-		msgToWaiter.AppendBlock(*blocks.NewBlock([]byte(expectedStr)))
+		msgToWaiter.AddBlock(*blocks.NewBlock([]byte(expectedStr)))
 
 		return fromWaiter, msgToWaiter
 	}))
@@ -105,7 +105,7 @@ func TestSendMessageAsyncButWaitForResponse(t *testing.T) {
 	}))
 
 	messageSentAsync := bsmsg.New()
-	messageSentAsync.AppendBlock(*blocks.NewBlock([]byte("data")))
+	messageSentAsync.AddBlock(*blocks.NewBlock([]byte("data")))
 	errSending := waiter.SendMessage(
 		context.Background(), peer.WithID(idOfResponder), messageSentAsync)
 	if errSending != nil {