Fortigate 30E "Conflicts with 'wan' subnet"

We have an issue with our Fortigate 30E we placed at a building with different tenants. We have got a /27 subnet with public ip adresses from the isp, tenants can use one of those ip adresses on their own router and then use the fortigate as gateway to access the internet. NAT is disabled on the fortigate.

The wan interface  of the fortigate is connected through pppoe and get's his IP adress automatically from the ISP. On the LAN interface we use that same subnet with public ip's. 

Everything works oke. But if we want to enable device detection on the lan interface we get an error saying that the ip subnet of the lan interface Conflicts with 'wan' subnet"

Should we use a different setup? Or is it safe to use the allow-subnet-overlap command?