THE RPTR BEING #1 TAXIED INTO POS AND HOLD WHEN THE CTLR INSTRUCTED; WITHOUT HEARING WHO THE INSTRUCTION WAS FOR. THE RPTR ACKNOWLEDGED WITH HIS CALL SIGN BUT THE CTLR DID NOT CATCH THE READBACK.

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AT APPROX XA55 LCL TWR CLRED AN ACFT INTO POS AND HOLD ON RWY 28L AT PIT. WE WERE THE #1 ACFT IN LINE FOR RWY 28L. FLT NUMBER XXXX; WE ACKNOWLEDGE 'POS AND HOLD XXXX' AND PROCEEDED INTO POS ON RWY 28L. THE TWR THEN CLRED XXXA FOR TKOF RWY 28L; THE XXXA FLT ACKNOWLEDGE THE TKOF BUT STATED THERE WAS AN ACFT ON THE RWY. WE THEN NOTIFIED THE TWR THAT XXXX WAS ON THE RWY TWR THEN ORDERED US TO CLR AND XXXA INTO POS AND HOLD. THE XXXA FLT WAS ASKED WHAT HE HEARD HE STATED 'WE HEARD SOMEBODY CLRED INTO POS AND HOLD WE THOUGHT IT WAS US BUT WE WERE NOT #1 IN LINE.' THE XXXA THEN TOOK OFF WE WERE CLRED ON THE RWY FOR TKOF NO FURTHER SIT DEVELOPED. THIS WAS WELL INTO OUR 12 HR OF DUTY FOR THE DAY OR 7TH LEG AND ALREADY 6 HRS AND 40 MIN OF FLT TIME. THE WX CONDITIONS FOR THE DAY WERE LOW IMC AND HOLDING FOR APCHS WAS OCCURRING. AT THIS POINT IN TIME THE CREW'S FATIGUE FACTOR WAS HIGH AND THOUGHTS WERE ON THE OVERNIGHT AT CHO. IT IS APPARENT THAT A MISCOM OCCURRED THE XXXA FLT NEVER ACKNOWLEDGED POS AND HOLD WE DID. THE TWR DID NOT CATCH THE WRONG READBACK NOR THE ACFT ON THE RWY IN POS.