Has a general aviation twin-engine turboprop with counter rotating propellors ever been built?
The only aircraft with turboprop counter rotating propellers that I can think of is the Piaggio P 180 Avanti, whose design is unusual in so many ways. 

Check out http://www.mpa.piaggioaerospace.it/, which gives the specifications of a Maritime Patrol Aircraft(MPA) based on the P 180.