CREW DID NOT HEAR OR READ BACK HOLD SHORT INSTRUCTION. ENTERED ACTIVE RWY.

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LANDED RWY 24 AT FOK (WEST HAMPTON; NY). CLRED TO TAXI TO RAMP; HOWEVER; CTLR EVIDENTLY INSTRUCTED US TO HOLD SHORT OF RWY 19 AND WE DIDN'T. SIT WAS DISCOVERED WHEN GND CTLR INSTRUCTED US TO STOP; AND REMINDED US WE WERE TOLD TO HOLD SHORT OF RWY 19. NO EVASIVE ACTION WAS REQUIRED BY ANYONE. IN RETROSPECT; MYSELF (PF) AND PNF BOTH AGREE WE BOTH MISSED THE HOLD SHORT INSTRUCTION. WE (PF AND PNF) BOTH AGREED THAT CTLR DID NOT INSTRUCT US TO READ BACK THE HOLD SHORT INSTRUCTION WHICH WE BELIEVE WAS A CONTRIBUTING FACTOR TO THE INCURSION; AND THAT WE DIDN'T READ BACK THE HOLD SHORT INSTRUCTION. WE (PF AND PNF) BOTH AGREED THE MAIN REASON CONTRIBUTING TO THE INCURSION WAS WE WERE BOTH FATIGUED. WE WERE INTO OUR 13TH HR OF 'OFFICIAL' DUTY; 7 HRS OF FLT; 7 FLT LEGS. WE WERE UP VERY EARLY THIS MORNING; AT LEAST SEVERAL HRS TO SHOWER; EAT; CHK OUT OF HOTEL; TRAVEL TO ARPT; FILE FLT PLANS; AND PREPARE ACFT FOR THE DAY'S TRIP PRIOR TO THE START OF THE OFFICIAL BEGINNING OF DUTY TIME. I BELIEVE DUTY TIME SHOULD RECOGNIZE THAT A PLT'S DAY BEGINS MUCH EARLIER THAN WHEN YOU JUST ARRIVE AT THE ARPT. IN OTHER WORDS SHORTEN THE DUTY DAY.