A CTR RADAR CTLR DSNDED A CGA CL60 ACFT TO AN OCCUPIED ALT RESULTING IN LOSS OF SEPARATION. THE CTLR STATES THAT TFC SATURATION CONTRIBUTED TO THE ERROR.

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ACR X NBOUND TO ORD ON J89 AT FL240. CGA Y SWBOUND ON GUITR1 TO BNA AT FL250. CGA Y WAS AT FL250 DUE TO AN SDF ARR AT FL240. WHEN THE SDF ARR CLRED FL240; I THEN DSNDED CGA Y TO FL240 OVERLOOKING ADDITIONAL TFC WHICH WAS ACR X. THERE WERE 30 DATA BLOCKS AT THE SECTOR. APPROX 20-25 ACFT ON FREQ. THE SECTOR STAFFING WAS AT A MAX; NON RADAR CTLR (D SIDE); MYSELF ON RADAR AND A HDOF CTLR (TRACKER). TFC WAS PEAKING FROM XP02-XP25 TO BRING THE SECTOR TO A (RED) BY THE FLOW CTL ASD PROCESS. DURING THIS TIME PERIOD; FREQ CONGESTION BECAME A MAJOR PROB. BTWN XP09 AND XP18Z THERE WERE 12 BLOCKS RESULTING IN HAVING TO REPEAT CLRNCS AND UTILIZING PRECIOUS TIME. AFTER CLRING CGA Y TO FL240 I OBSERVED THE CONFLICT WITH ACR X APPROX 6 MI APART (CONFLICT ALERT HAD NOT ACTIVATED YET). I ISSUED A 270 DEG HDG TO ACR X AND THEN CLRED CGA Y TO FL200 WITH AN EXPEDITIOUS DSCNT THROUGH FL230. ACR X THEN RESPONDED TO HIS TCASII AND DSNDED AS WELL WHICH RESULTED IN A CONTINUING LOSS OF SEPARATION. SUPPLEMENTAL INFO FROM ACN 398088: THE SECTOR I WAS WORKING IS A SEQUENCING SECTOR FOR CVG ARRS. THE WAY THE AIRSPACE IS FLOWED AND ARR/DEP RTES ARE MANDATED GIVES VERY LITTLE ROOM FOR ERROR. THE REASON ACFT X AND Y GOT TOGETHER WAS TOO MANY AIRPLANES IN A CONFINED; INFLEXIBLE SECTOR. ACFT X AND ACFT Y WERE OVERLOOKED. MY CONCERN IS THAT THIS SECTOR'S OVERSATURATION IS A DAILY OCCURRENCE. SUPPLEMENTAL INFO FROM ACN 398417: TCASII TA FOR ACFT 800 FT ABOVE; DSNDING APPROX 2 O'CLOCK; 10 NM. THEN TCASII DSND RA FOR ACFT 600 FT ABOVE; DSNDING APPROX 3 O'CLOCK; 8 NM. CAPT DISCONNECTED AUTOPLT AND INITIATED 1800 FPM DSCNT IN 1500-2000 FPM IVSI GREEN TCASII ARC. TCASII INCREASE DSCNT RA FOLLOWED AND WAS COMPLIED WITH. COPLT ESTIMATES .25 NM SEPARATION BTWN ACFT AS OTHER ACFT DSNDED THROUGH OUR ALT BEHIND US. DURING THIS TIME ATC GOT ACKNOWLEDGEMENT BY OTHER ACFT OF VISUAL CONTACT WITH OUR ACFT -- ATC DID NOT ADVISE US OF OTHER ACFT. LOWEST ALT DURING MANEUVER WAS FL228.