Error about installs files into the same place. cups-client and cups-image are installed earlier by some port within x11/kde4 meta-port dependency.
Can you be more specific? What exactly does the error say?
(In reply to Raphael Kubo da Costa from comment #1) Could you please have a try to compile x11/kde4 on your side to see its details? Can't copy/paste the whole output from virtualbox for now. Got the latest current base and ports. Thank you.
Please read the 20160311 entry in /usr/ports/UPDATING about updating CUPS.