MLG COPIED CLRNC TO DESCEND MEANT FOR LGT WITH SAME A/N AND HAD LESS THAN STANDARD SEPARATION WITH ANOTHER LGT OPPOSITE DIRECTION. CTLR MISSED INCORRECT READBACK AND HAD NOT ADVISED LGT AND MLG OF SAME A/N SITUATION.

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I WAS ONE-MANNING THE SOMERSET SUPER HIGH SECTOR AT INDIANAPOLIS ARTCC. I WAS WORKING BOTH THE RADAR AND MANUAL POSITION DUE TO STAFFING LIMITATIONS. I HAD 10 AIRPLANES ON THE FREQ. BY THE WAY; MY AIRSPACE WAS FL350 AND ABOVE; ONE OF THE AIRPLANES WAS DEVIATING AROUND WEATHER; AND 4 OTHER ACFT WERE ON VECTORS; ONE OF THE VECTORED ACFT WAS LGT X (ABC) THE OTHER 3 ACFT BEING VECTORED WERE IN THE NORTHERN REGION OF MY AIRSPACE. I WAS TAKING A NBOUND ACR FLT AT FL350 BETWEEN TWO WBOUND ACR FLTS AT FL350. IT WAS A ROUTINE MANEUVER; BUT I DIDN'T HAVE MUCH ROOM FOR ERROR. THE TWO WBOUND FLTS WERE ONLY 15 MILES IN TRAIL. MLG Y (ABC) CALLED ON AT FL350. I DID NOT NOTICE THE SIMILAR CALL SIGN WITH LGT X (ABC); AND NEITHER FLT WAS INFORMED ON THE SIMILAR CALL SIGNS. I NEEDED TO DESCEND LGT X (ABC) TO AVOID MY ACFT DEVIATING AROUND THE WX. THE CTLR BELOW ME APPROVED DESCENT TO FL 240. I CLRED LGT X (ABC) TO FL240. MLG Y (ABC) ACKNOWLEDGED THE CLRNC. I MISSED THE READBACK. 20 SECONDS LATER LGT X (ABC) REQUESTED LOWER. I QUESTIONINGLY (I DON'T THINK THAT'S A WORD) RE-CLRED LGT X (ABC) TO FL240. MY 'FIELD GOAL' STILL HAD MY ATTN. WHEN CONFLICT ALERT ACTIVATED BETWEEN MLG X (ABC) AND LGT Z; SBOUND AT FL330. MLG Y (ABC) WAS OUT OF FL344; AND GOT DOWN TO FL337; BEFORE I TOLD HIM TO CLIMB TO FL350; THAT HE HAD TAKEN ANOTHER ACFT'S CLRNC. I TURNED HIM DUE WEST AND CALLED HIS TFC; ASKED HIM TO REPORT LEVEL AT FL350. HE REPORTED THE TFC IN SIGHT AND LEVEL AT 350; 9 SECONDS AFTER OED ACTIVATED. THE CLOSEST THEY GOT WAS 2.6 MILES AND 1500'. SUPPLEMENTAL INFORMATION FROM ACN 86709: A SHORT TIME LATER THE CTLR TOLD US THAT CONFLICT BELL HAD GONE OFF AND WE COULD CALL QUALITY ASSURANCE ON THE GND. WHICH I DID. AFTER REVIEWING THE TAPES IT WAS CONCLUDED THAT THE CTLR HAD CLRED LGT X (ABC) - TO FL240 BUT DIDN'T CATCH THE FACT THAT WE RESPONDED TO THE CALL. LGT X (ABC) DIDN'T RESPOND TO THE INITIAL CALL BUT ASKED FOR A LOWER AT THAT SAME TIME; THAT'S WHEN THE CTLR SEEING US DESCENDING REALIZED HIS ERROR. THE QUALITY ASSURANCE PEOPLE WERE VERY RESPONSIVE TO THE PROBLEM THEY FILED A CTLR OPERATIONAL ERROR FOR FAILURE TO ADVISE OF LIKE CALL SIGNS; AND NOT CATCHING THE WRONG READ BACK. ACCORDING TO QUALITY ASSURANCE A PLT DEVIATION WAS NOT FILED.