Or you have a loop.
I had weird things a few years ago where I hooked a router wan back to a switch. The switch was on the WAN side, so that made things crazy!
That is related to "Loops and Spanning Trees" and only hi-end routers/switches can usually safeguard for that.
Most home networks are simple enough that this is unlikely. I had about 12 headless LINUX machines under my stairs booting from the network. It was a messy setup.
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