IP Conflict

Hello all; I am having a problem with a small network that I need help with, this has happened a few times now where two laptops receive the same IP address from my DHCP server, and it is always the same 2 laptops, I was able to fix this momentarily by excluding the IP address that was in question then both laptops received a different IP but then after a week the same problem occurred, I don’t why this keeps occurring with the same 2 laptops it makes no sense to me.
I now have 3 IP addresses excluded from my range because of this problem, the list will eventually get too big unless I fix the problem.

Thanks in advance for your help, if you need any further info please let me know.

CG._