SNMP Trap New Device Found

[Grog093]

We are using SNMP traps to alert when a new device is connected to an interface. Trap is going to PRTG and we can see the interface & mac address of the device fine however we are getting multiple alerts for the same mac addresses on the same interface each day. The idea was to use this to alert when either a device that has not been connected in a while or a new device is connected so we can confirm what the device is. 

Any ideas on why this is the case? I haven't defined the discovery timeout settings, default should be 28 days but do you think manually defining will help?