You are right, there would usually be "KERN_ARP_DUPLICATE_ADDR" or "KERN_ARP_ADDR_CHANGE" on the SRX itself. It is a weird problem for sure.
I would still go for the network capture, leave it running until the issue reappears and then have a look, at least if you see no "duplicate use of IP detected" when looking in Wireshark or whatever after the issue appears, you can probably rule out duplicate IPs actually in use on the network.
If this is the case, as to why the clients are presenting the error may be something else to look into.