The packetsDuplicated property of the RTCInboundRtpStreamStats dictionary indicates the total number of packets discarded because they were duplicates of previously-received packets.

These packets are not counted by the packetsDiscarded property.

An integer value which specifies how many duplicate packets have been received by the local end of this RTP stream so far. These duplicate packets are not included in the packetsDiscarded property.

Duplicate packets are detected when a packet has the same RTP sequence number as another packet that has previously been processed. Each time a packet is repeated, the value of packetsDuplicated is incremented, even if the same packet is received more than twice.

You can get a more accurate tally of how many packets have been lost on the stream by adding packetsDuplicated to packetsLost. The resulting value will be positive, although it will not match the count as computed in RFC 3660.