dhcp strange behavior : un authorized users can access network

I have MAC/IP binded network. Where devices have unique IP address. Only users with MAC/IP combination in dhcp.conf should be able to access internet.

But recently we have noticed, if new devices connected to our WiFi and if dhcp assigned random IP which is already assigned to MAC/IP, that new device can access internet.

/etc/dhcp/dhcpd.conf File has

```

subnet 192.168.1.0 netmask 255.255.255.0 { range 192.168.1.10 192.168.1.254;option domain-name "dhcp.local";option routers 192.168.1.1; option broadcast-address 192.168.1.255; max-lease-time 7200;option domain-name-servers 192.168.10.1;}

subnet 192.168.2.0 netmask 255.255.255.0{} subnet 192.168.3.0 netmask 255.255.255.0{}

include "/etc/dhcp/dhclient.d/dhcp_clients.conf";

allow booting; allow bootp; option option-128 code 128 = string; option option-129 code 129 = text; ```