diff --git a/ackhandler/sent_packet_handler.go b/ackhandler/sent_packet_handler.go
index 2a1a47b6..e8329c75 100644
--- a/ackhandler/sent_packet_handler.go
+++ b/ackhandler/sent_packet_handler.go
@@ -24,7 +24,7 @@ var (
 	errAckForUnsentPacket        = qerr.Error(qerr.InvalidAckData, "Received ACK for an unsent package")
 )
 
-var errDuplicatePacketNumber = errors.New("Packet number already exists in Packet History")
+var errPacketNumberNotIncreasing = errors.New("Already sent a packet with a higher packet number.")
 
 type sentPacketHandler struct {
 	lastSentPacketNumber protocol.PacketNumber
@@ -121,7 +121,9 @@ func (h *sentPacketHandler) queuePacketForRetransmission(packetElement *ackhandl
 }
 
 func (h *sentPacketHandler) SentPacket(packet *ackhandlerlegacy.Packet) error {
-	// TODO: check for decreasing or duplicate packet numbers
+	if packet.PacketNumber <= h.lastSentPacketNumber {
+		return errPacketNumberNotIncreasing
+	}
 
 	now := time.Now()
 	h.lastSentPacketTime = now
diff --git a/ackhandler/sent_packet_handler_test.go b/ackhandler/sent_packet_handler_test.go
index b9c92d75..322a44c2 100644
--- a/ackhandler/sent_packet_handler_test.go
+++ b/ackhandler/sent_packet_handler_test.go
@@ -93,18 +93,30 @@ var _ = Describe("SentPacketHandler", func() {
 			Expect(handler.BytesInFlight()).To(Equal(protocol.ByteCount(3)))
 		})
 
-		PIt("rejects packets with the same packet number", func() {
+		It("rejects packets with the same packet number", func() {
 			packet1 := ackhandlerlegacy.Packet{PacketNumber: 1, Frames: []frames.Frame{&streamFrame}, Length: 1}
 			packet2 := ackhandlerlegacy.Packet{PacketNumber: 1, Frames: []frames.Frame{&streamFrame}, Length: 2}
 			err := handler.SentPacket(&packet1)
 			Expect(err).ToNot(HaveOccurred())
 			err = handler.SentPacket(&packet2)
-			Expect(err).To(MatchError(errDuplicatePacketNumber))
+			Expect(err).To(MatchError(errPacketNumberNotIncreasing))
 			Expect(handler.lastSentPacketNumber).To(Equal(protocol.PacketNumber(1)))
 			Expect(handler.packetHistory.Front().Value.PacketNumber).To(Equal(protocol.PacketNumber(1)))
 			Expect(handler.BytesInFlight()).To(Equal(protocol.ByteCount(1)))
 		})
 
+		It("rejects packets with decreasing packet number", func() {
+			packet1 := ackhandlerlegacy.Packet{PacketNumber: 2, Frames: []frames.Frame{&streamFrame}, Length: 1}
+			packet2 := ackhandlerlegacy.Packet{PacketNumber: 1, Frames: []frames.Frame{&streamFrame}, Length: 2}
+			err := handler.SentPacket(&packet1)
+			Expect(err).ToNot(HaveOccurred())
+			err = handler.SentPacket(&packet2)
+			Expect(err).To(MatchError(errPacketNumberNotIncreasing))
+			Expect(handler.lastSentPacketNumber).To(Equal(protocol.PacketNumber(2)))
+			Expect(handler.packetHistory.Front().Value.PacketNumber).To(Equal(protocol.PacketNumber(2)))
+			Expect(handler.BytesInFlight()).To(Equal(protocol.ByteCount(1)))
+		})
+
 		It("works with non-consecutive packet numbers", func() {
 			packet1 := ackhandlerlegacy.Packet{PacketNumber: 1, Frames: []frames.Frame{&streamFrame}, Length: 1}
 			packet2 := ackhandlerlegacy.Packet{PacketNumber: 3, Frames: []frames.Frame{&streamFrame}, Length: 2}