[jammies]

To be a bit more helpful: try rebooting every device connected to your network, including phones, printers, and so on. Sometimes, rather than a static IP being the issue, a device doesn't properly renew its IP and the router reissues that IP while its still in use. Rebooting devices forces them to request a renewal, and thus will often fix the issue.

Not to go into too much detail, but the reason the addresses need to be "renewed" is that you don't want to have a device you once had, but no longer do, to keep its address forever, as eventually you might run out. So every few days, the devices are supposed to say "hey dude, I'm still here, can I keep this IP address?" and the router/server says "No problemo! Just let me update my records that you're still around!" If a device doesn't check in, and the address' validity expires, the router/server assumes that device isn't around anymore and that its address is free to give to some other device. Which is where a duplicate comes from.