Duplicate Pings Causing Problems

Two homes linked with a zeroTier network. Ubuntu VM with v.1.12.2 at one location acting as router, Pi4 at the other location also running 1.12.2. Both of these routing devices had earlier versions of the zeroTier client, but I upgraded them as part of trying to resolve this problem.

The configuration has been working more or less trouble-free for about 3 years. Recently while trying to diagnose an unrelated inter-lan connectivity issue, I noted that pings result in duplicates. I first noticed this when pinging random nodes on one network from nodes on the other network.

I then moved on to the routing device on each network, tried pinging one from the other using the zeroTier address - this also produces DUPs. So, something in the zeroTier configuration is resulting in DUPs on pings when pinging using the zeroTier address space from one node on the network to the other. I do not recall changing anything recently, and unfortunately, I have no information on when this behavior started, since I usually don’t have any reason to ping stuff.

Using ping -I to explicitly specify the zeroTier interface produces the same result.

Any suggestions on how to investigate this further? I saw some mention of DUPs several versions ago, but I thought that was fixed.

The presence of the duplicate pings is preventing a new application from working properly because it relies on a simple single ping response to monitor connectivity. The DUPs cause it to fail.