CVG CTLR DESCRIBED POTENTIAL CONFLICT WITH INBOUND HELI AND DEP ACFT ON RWY 18L/18C OPERATION; LISTING FAILED HEARBACK AS CAUSAL FACTOR.

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I INSTRUCTED ACFT X; A BELL RANGER INBOUND TO CVG; TO REMAIN E OF THE RWY 18L CTRLINE. THE PLT READ BACK 'REMAIN E OF THE RWY 18 CTRLINE.' I CONTINUED TO CLR ACFT TO LAND AND TAKE OFF FROM RWY 18L. ACFT X ASKED IF THEY WERE CLRED INTO THE CLASS B AIRSPACE. I ISSUED CLRNC INTO THE CLASS B AND REITERATED; WHAT I THOUGHT TO BE; INSTRUCTIONS TO REMAIN E OF RWY 18L. I HAD ACFT Y; A CRJ; WITH A FLOW TIME TO MEET AT RWY 18L. I INSTRUCTED THAT ACFT TO POS AND HOLD ON RWY 18L. WHEN THE PRECEDING ARR CLRED THE RWY; I INFORMED THE CRJ OF THE JET RANGER TFC AND THAT IT WAS REMAINING E OF THE RWY; THEN CLRED HIM FOR TKOF. AT THAT VERY MOMENT; I OBSERVED ACFT X FLYING ACROSS THE RWY AT THE T6 INTXN ENRTE TO THE FBO. THERE WAS NO TIME TO CANCEL ACFT Y'S TKOF CLRNC. ACFT X WAS ACROSS THE RWY AND GOING DOWN TXWY M BEFORE ACFT Y BEGAN HIS TKOF ROLL. I WAS SO SHAKEN BY THE INCIDENT THAT I INFORMED ACFT X TO CALL THE TWR AND FAILED TO ISSUE A LNDG CLRNC OF WHICH THE PLT LANDED WITHOUT. THE PLT INFORMED ME THAT THEY HEARD 'REMAIN E OF RWY 18C.' I IMMEDIATELY ASKED TO BE RELIEVED FROM POS SO I COULD REVIEW THE TAPES. I HAD ORIGINALLY ISSUED 'REMAIN E OF RWY 18L' THEN WHEN THE PLT ASKED FOR CLASS B CLRNC; I MISTAKENLY RECLRED THE PLT TO REMAIN E OF RWY 18C AND MISSED THE READBACK OF RWY 18C.