BEECH 1900 CAPT BELIEVED THAT HE HEARD HIS CALL SIGN CLRED FOR TKOF AND REPEATED BACK SAME AS HE TAXIED INTO POS TO TKOF.

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MY FO AND I WERE LEAVING PHL ON AN UNPLANNED FERRY FLT AT THE END OF WHAT WAS OTHERWISE A NORMAL SHIFT. IT WAS XA50. WE WERE STILL FACING SEVERAL MORE HRS OF DUTY FERRYING OUR ACFT FROM PHL TO DUJ; WAITING FOR REPAIRS; AND REPOSITIONING TO RDG WHERE WE WOULD HAVE NORMALLY FINISHED OUR SHIFT. GND CTL CLRED US TO TAXI TO RWY 35 INTXN K; WHICH IS THE MOST COMMON DEP POINT ON THAT RWY. ONCE REACHING THE HOLD SHORT LINE; I COMPLETED SEVERAL BEFORE TKOF CHKLIST ITEMS WHILE MY FO REVIEWED OUR CLRED RTE; AS IT WAS ONE WE WERE NOT ACCUSTOMED TO. I WAS WORKING THE RADIOS WHILE HE HAD HIS HEAD DOWN IN HIS CHARTS. ANOTHER ACFT WAS HOLDING SHORT AT THE END OF RWY 35 READY FOR DEP. SHORTLY AFTER ARRIVING AT THE RWY; I BELIEVE I HEARD THE TWR CTLR SAY 'ACR X; RWY 35 INTXN K; CLRED FOR TKOF.' I READ BACK THE INSTRUCTION AND BEGAN TO TAXI INTO POS. SHORTLY AFTER THAT; THE ACFT AT THE END OF RWY 35 ASKED TWR WHO IT WAS AT INTXN K. TWR REPEATED THE QUESTION TO US; AND I REPLIED 'THIS IS ACR X; YOU JUST CLRED US FOR TKOF.' AT THAT POINT TWR REPLIED; 'I CLRED (THE OTHER ACFT CALL SIGN) FOR TKOF; BUT STATED INTXN K INSTEAD OF FULL LENGTH.' TWR APOLOGIZED AND CLRED US FOR TKOF. ALTHOUGH TWR ADMITTED FAULT; IT WAS OBVIOUSLY BOTH OF OUR MISTAKES THAT LED TO THE INCURSION. I THOUGHT I HEARD OUR CALL SIGN; BUT CERTAINLY CAN'T ATTEST TO THAT WITH ANY CERTAINTY. I AM SURE HE SAID INTXN K AND; SINCE WE WERE THE ONLY ACFT HOLDING SHORT AT THAT INTXN; I ASSUMED IT HAD TO BE US. THE FACTORS INVOLVED IN THIS INCURSION ARE PROBABLY ALL OF THE MOST STANDARD ONES: FATIGUE; HASTE; DEVIATING FROM OUR NORMAL FLOW (WORKING THE RADIOS AND TAXIING WHILE MY FO REVIEWED THE RTE); DARKNESS (TWR COULDN'T IDENT US POSITIVELY) AND; OF COURSE; COMPLACENCY. REMOVING ANY ONE OF THESE FACTORS WOULD HAVE PREVENTED THE INCURSION.