So I reinstalled Ubuntu server and set the ip to 204, but I forgot that I assigned a static ip to it in my dhcp. I guess it has two IPv4 addresses now? How? Only one NIC 🤔

Thats probably the leases only

You can have as many addresses as you want, but yes it's weird because startup scripts usually opt between static and dynamic.
Remove one with ip addr del (man iproute2) or check config and reboot.
If it's a scanner, could be just arp cache

Your NIC will just respond to both addresses. With IPv6 you typically have at least two addresses per NIC.