OS X Mavericks :: Another Device On Network Is Using IP Address

a macbook air and an iMac have both give the error "Another device on the network is using your IP address". They are both using the latest OSX and are both set to use DHCP assigned by my Virgin cable router.the address reported in the popup window is NOT the address that either computer was assigned at the time (the first time, I thought it was odd, and I had an iPad handy and used fing to search my network to see what device had just used that address and I didn't see anything). This time - I was on my iMac and happen to have a terminal open, so I did a quick if config - and my ip address was not the one indicated in the message (it was saying 192.168.0.8) and my address was 192.168.0.12. In fact I think that the other day it was complaining about the same .0.8 address.

I do have 2 devices that have a hard coded ip-address (but they are right at the beginning of the range, and in fact I have instructed the router to skip those addresses and just dish out ip's from .0.3 onwards). All other devices use DHCP. 

Info:
iMac, OS X Mavericks (10.9.4)