I have a linksys router and are always fiddeling with my Dreamboxes and other brands.
I run into this issues very often and it's always my router who is the cause.
In your router check the device list and as Cepheus62 mention check your dhcp assignment.
Clear out the device list to get ride of old saved devices. Check your dhcp settings and check if mask mac address is activated or in some cases even clone mac address can be an option.
The clear out device list always fix my issues when i run into double ipaddress