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I have understood that real networks have routing changes, leading to out-of-order delivered IP packets. Real networks also have frequent packet loss.

However, what I have not understood is why could a perfectly operating network in addition to out-of-order and lost packets deliver packets as duplicates. What could cause delivery of such duplicate packets?

Could spanning tree protocol (STP) before converging on a spanning tree cause duplicate packets?

Note that I don't currently have a network that would frequently duplicate packets, so this is more of a theoretical question than a practical one.

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Real networks also have frequent packet loss.

If a network suffers frequent packet loss it is either badly designed or its load has outgrown its capacity. Packet loss does appear in almost any network but should usually be below 1 ppm.

(M/R)STP can cause duplicate packets when it's not configured correctly. Fast start should not be used on inter-switch links as it effectively causes loops for short periods.

Additionally, various link aggregation misconfigurations can cause duplicates, especially with "static", non-protocol LAGs.

Also, bloated router buffers can cause duplicates when the sender starts resending TCP segments that are still buffered on the router. Unbalanced links with different latencies and load-balancing can very easily cause this.

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I have understood that real networks have routing changes, leading to out-of-order delivered IP packets.

Out of order packets are usually caused by multiple links of different latency, rather than routing changes.

Network devices like routers and switches forward packets from one interface to another. They don't duplicate packets, so it's not the network device that causes duplicate packets. More commonly, the sender does not get an acknowledgement in time (perhaps due to a network change), and then sends a duplicate packet.

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