CAPT OF AN MDT ABORTED TKOF DUE TO A TKOF WARNING HORN INDICATING THAT THE ACFT WAS NOT CONFIGURED FOR TKOF.

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DURING TAXI; THE CAPT CALLED FOR THE BEFORE TKOF CHKLIST. I HAD JUST STARTED MY 'BEFORE TKOF' FLOW PATTERN WHEN THE FLT ATTENDANT CAME UP TO RPT A PAX PROB. I FELT I HAD COMPLETED THE BEFORE TKOF FLOW CORRECTLY. I THEN STARTED TO READ THE CHKLIST TO VERIFY THE ITEMS WHEN THE TWR INTERRUPTED; 'CLRED FOR TKOF WITHOUT DELAY; TFC SHORT FINAL.' I RESPONDED; 'ROGER; CLRED FOR TKOF.' WE HAD TO SOLVE A PAX PROB; AND PREPARE FOR TKOF. I THEN COMPLETED READING AND VERIFYING THE REMAINDER OF THE CHKLIST ITEMS. IT WAS NIGHTTIME AND VERY DARK IN THE COCKPIT. WE APPARENTLY MISSED THE FACT THAT THE CONDITION LEVERS WERE NOT ALL THE WAY FORWARD DURING THE CHKLIST. I KNOW THAT DISTR; DARKNESS; PARALLAX; AND FATIGUE ALL LED TO OUR NOT NOTICING THE EXACT POS OF THE CONDITION LEVERS. AS WE STARTED THE TKOF RUN; A TKOF WARNING HORN SOUNDED. THE CAPT ABORTED THE TKOF AND PULLED OFF THE RWY. WE PERFORMED THE CHKLIST ITEMS AGAIN. WE DISCOVERED THE DISCREPANCY (CONCERNING THE CONDITION LEVER POS); ELECTED TO RETURN FOR TKOF; AND DEPARTED WITHOUT INCIDENT. I DO KNOW THAT IT IS RECOMMENDED TO RESTART A FLOW PATTERN OR RESTART A CHKLIST AFTER AN INTERRUPTION OR DISTR. I KNOW NOW THAT I WILL DO IT EVERY TIME WITHOUT EXCEPTION; PHYSICALLY TOUCH THE CONDITION LEVERS DURING THE CHKLIST; AND DECLINE A TKOF CLRNC UNTIL ALL PROBS ARE RESOLVED.