S80 FLC ACKNOWLEDGE CLRNC TO TAXI INTO POS AND HOLD AT ORD RWY 4L; BUT CAPT DOUBTS CALL SIGN USAGE.

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#1 AT RWY; ORD TWR SAID 'ACR X; (I WAS NOT QUITE SURE WHAT NUMBER; BUT OF COURSE EXPECTED TO HEAR ABCD) TAXI INTO POS AND HOLD RWY 4L; BE READY FOR IMMEDIATE TKOF.' THE FO ACKNOWLEDGED 'ACR X ABCD POS AND HOLD RWY 4L.' SINCE THE TWR WAS VERY BUSY COMMUNICATING WITH RWY 9L LNDGS; RWY 32L AND RWY 4L TKOFS; I ASSUMED HE GAVE OUR FLT NUMBER CORRECTLY. RWY 4L WAS ONLY BEING USED FOR TKOFS; SO THERE WAS NO POSSIBLE CONFLICT WITH OTHER TFC. AFTER TAKING POS; ORD TWR SAID 'ACR X (A DIFFERENT FLT NUMBER); CLRED FOR IMMEDIATE TKOF RWY 4L.' WE CORRECTED HIM ON OUR CORRECT FLT NUMBER AND THEN HE CLRED US; ACR X ABCD; FOR AN IMMEDIATE TKOF RWY 4L. THERE WAS NEVER ANY REAL PROB AND THE FO TOLD ME LATER WHEN I ASKED ABOUT THE SIT; THAT HE WAS CERTAIN THE TWR USED OUR CORRECT CALLSIGN WHEN CLRING US INTO POS. I DON'T KNOW FOR SURE WHETHER OR NOT THE CORRECT CALL SIGN WAS USED; BUT HAVE SERIOUS DOUBT AFTER HE CLRED THE WRONG AIRPLANE FOR TKOF; WHICH TURNED OUT TO BE THE ACFT BEHIND US IN LINE. I DON'T KNOW HOW MANY TIMES I HAVE READ ABOUT SITS LIKE THIS AND DIDN'T THINK I COULD FALL INTO THE TRAP OF ACCEPTING A CLRNC I DIDN'T HEAR CLRLY -- BUT I DID. I GUESS IT WAS MY DESIRE TO HELP KEEP THINGS MOVING WHICH SHOULDN'T HAVE BEEN A FACTOR AND WONT' AGAIN.