A COMMERCIAL FIXED WING LTT WAS DISPATCHED WITH THE WRONG AND INCORRECT DISPATCH RELEASE.

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WE HAD A NEW HIRE FO WHO WAS GOING TO RIDE JUMP SEAT WITH US FOR OBSERVATION PURPOSES. HE MET US ON THE AIRPLANE SO I TOLD HIM TO STAY WITH MY SENIOR FO AND OBSERVE HIS PREFLT DUTIES WHILE I WENT INTO OPS TO PICK UP MY DISPATCH RELEASE AND HIS JUMP SEAT PASS. I PICKED UP MY DISPATCH RELEASE OUT OF ITS CUBBY HOLE; LOOKED IT OVER; AND SIGNED IT. THEN I CALLED FLT CTL TO REQUEST A JUMP SEAT PASS. DISPATCH TOLD ME NOT TO USE MY OLD RELEASE BUT RATHER THEY WOULD SEND ME A NEW ONE WITH THE JUMP SEAT RIDER LISTED ON IT. USUALLY WE JOINTLY AMEND THE RELEASE AND PIN THE JUMP SEAT RIDER'S NAME; SO I WAITED FOR MY NEW RELEASE. ONE WAS PUT IN MY DEST'S CUBBY HOLE. I PICKED IT UP; CHKED IT FOR MY JUMP SEAT RIDER'S NAME; SIGNED; AND OFF I WENT. I THREW MY ORIGINAL RELEASE AWAY. ABOUT 5 MINS INTO THE FLT; I REALIZED I HAD SIGNED A RELEASE WITH THE WRONG FLT NUMBER AND CAPT; SAME CITY AND FUEL AND WITH MY JUMP SEAT RIDER'S NAME. THE DISPATCHER HAD SENT ME THE WRONG RELEASE WITH THE RIGHT JUMP SEATER'S NAME; AND CORRECT CITY; FUEL; AND RTE. WHEN I ARRIVED AT MY DEST; I REQUESTED AN AMENDED RELEASE FOR THE RETURN FLT HOME. FACTORS: 1) FATIGUE -- 12 LEG TRIP WITH A SCHEDULED REDUCED REST REMAIN OVERNIGHT OF 8 HRS 20 MINS. THIS WAS THE LAST ROUND TRIP. THE PREVIOUS TRIP WAS ALSO A TWO DAY TRIP WITH 12 LEGS! 2) HURRY -- WE HAD A QUICK TURN TIME BECAUSE OF ATC; WX DELAYS; AND SHORT SCHEDULED TURN TIME. THIS WAS A FACTOR BECAUSE I CHKED MY FIRST RELEASE 'CAREFULLY;' BUT DIDN'T SPEND THE TIME TO CHECK MY SECOND RELEASE EXCEPT FOR THE JUMP SEAT RIDER. ALSO; DISPATCH PUTS 2 OR 3 RELEASES FOR THE SAME CITY; DIFFERENT TIMES; IN THE SAME CUBBY HOLE. IT'S EASY TO PICK UP A RELEASE FOR YOUR CITY; BUT LATER ON THAT DAY; ESPECIALLY WHEN YOU ARE HURRYING. 3) DISTRS -- AS I WAITED FOR MY NEW RELEASE; SEVERAL PLTS CAME UP TO ME AND STARTED GENERAL CONVERSATION. THIS WASTED SOME VALUABLE TIME I COULD HAVE BEEN STUDYING MY NEW DISPATCH RELEASE. I STRIVE TO HAVE A CONSISTENT PATTERN OF CHKING MY RELEASES AND MEL ITEMS; HOWEVER; THIS TIME THERE WAS AN ABNORMAL SIT AND COMPOUNDED WITH THE ABOVE THREE FACTORS I MADE AN ERROR. THE SERIES OF BACKUP CHKS FAILED TO CATCH IT.