I have just discovered that my Raspbian PI have both dhcpcd and dhclient installed and in use. And I know I've not installed either so I'm inclined to believe that they are both active in the installation kit.
The machine works fine with this setup until one wants to make special changes such as adding a DNS server which is not supplied by DHCP. Then it becomes weird.
It might be a good idea to disable one or the other. But which one?