TWR LCL CTLR CONFUSES 2 SAME COMPANY ACFT WITH SIMILAR CALL SIGNS HOLDING FOR TKOF. THE ONE NOT HOLDING IN POS IS CLRED FOR TKOF AND THE ONE IN POS AND HOLDING INFORMS THE CTLR OF THE MISTAKE. THE CTL THEN MAKES THE CORRECTION AND ISSUES NEW TKOF INSTRUCTIONS TO THE ONE HOLDING IN POS.

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AT IAH ON MAR/XA/99 WE WERE FLT ACR X. WE WERE DIRECTED BY THE GND CTLR TO TAXI TO THESE POS AND MONITOR TWR. BOTH FLTS REMAINED AT THESE POS FOR SEVERAL MINS WHILE OTHER FLTS DEPARTED RWY 26. THEN TWR SAID; 'ACR X MAKE THE L TURN; TAXI UP TO AND HOLD SHORT OF RWY 26;' WHICH WE DID. THEN TWR SAID EITHER' ACR X OR ACR Y (WE ARE NOT SURE) RWY 26 POS AND HOLD; BE READY FOR AN IMMEDIATE; TFC 3.5(?) MI FINAL.' WE TAXIED INTO POS ON RWY 26. THEN TWR SAID; 'ACR Y TURN R 020 DEGS; RWY 26 CLRED FOR TKOF.' WE THEN ASKED THE CTLR TO VERIFY OUR FLT NUMBER; AGAIN HE RESPONDED 'ACR Y.' WE TOLD HIM WE WERE ACR X HOLDING IN POST RWY 26. HE RESPONDED; 'OK ACR X; TURN R HDG 360 DEGS; RWY 26; CLRED FOR TKOF;' WHICH WE DID. SOLUTION: OUR FLT NUMBERS NEED TO BE MONITORED TO AVOID CONFUSING SITS LIKE THIS. I FEEL THAT WE AND THE CTLR WERE CONFUSED ABOUT THE 2 DIFFERENT FLT NUMBERS. ALSO WITH RWY 26; WE WERE HEARING A LOT OF 6'S. I ALSO BELIEVE THAT THE LNDG FLT HAD A 6 IN IT ALSO; BUT COULD NOT FIND FLT TO IDENT IT.