ignore duplicate packets

fixes #197

session.go

@@ -240,6 +240,11 @@ func (s *Session) handlePacketImpl(remoteAddr interface{}, hdr *publicHeader, da
 	}
 
 	err = s.receivedPacketHandler.ReceivedPacket(hdr.PacketNumber, packet.entropyBit)
+	// ignore duplicate packets
+	if err == ackhandlerlegacy.ErrDuplicatePacket {
+		return nil
+	}
+
 	if err != nil {
 		return err
 	}

session_test.go

@@ -426,6 +426,14 @@ var _ = Describe("Session", func() {
 			Expect(err).ToNot(HaveOccurred())
 			Expect(session.lastRcvdPacketNumber).To(Equal(protocol.PacketNumber(3)))
 		})
+
+		It("ignores duplicate packets", func() {
+			hdr.PacketNumber = 5
+			err := session.handlePacketImpl(nil, hdr, nil)
+			Expect(err).ToNot(HaveOccurred())
+			err = session.handlePacketImpl(nil, hdr, nil)
+			Expect(err).ToNot(HaveOccurred())
+		})
 	})
 
 	Context("sending packets", func() {