ZID CTLR LOST SEPARATION WITH 2 ACFT AT FL310 BECAUSE OF FAULTY AVOIDANCE VECTORS.

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WHILE WORKING THE RADAR ASSOCIATE POS; I NOTICED ACFT #1 IN CONFLICT WITH ACFT #2. I COORDINATED WITH THE APPROPRIATE FACILITIES TO GET CTL TO TURN BOTH ACFT; AND ADVISED THE RADAR CTLR HE COULD TURN EACH ACFT; AS NEEDED; TO MAINTAIN SEPARATION. THE RADAR CTLR TURNED ACFT #1 5 DEGS TO THE R. HE ALSO TURNED ACFT #2 TO A 290 DEG HDG. THE SIT WAS WELL UNDER CTL AND APPEARED TO BE WORKING. I THEN HEARD THE RADAR CTLR ASSIGN ACFT #1 A 215 DEG HDG; THEN HE STATED HE WAS GOING TO TURN 10 MORE DEGS TO THE R FOR ACFT #1 TO GIVE EVEN MORE SPACE; BUT IN DOING SO; THE RADAR CTLR TURNED ACFT #1 35 DEGS BACK INTO ACFT #2. THE DRASTIC L TURN APPEARED AT FIRST TO BE A FALSE INDICATION TO ME; AND I TOLD THE RADAR CTLR IT DIDN'T LOOK GOOD. ON THE SECOND RADAR UPDATE; I AGAIN TOLD THE RADAR CTLR THE SIT DIDN'T LOOK GOOD AND HE SHOULD DO SOMETHING. THE THIRD RADAR UPDATE LOOKED VERY BAD AND AS I SPOKE UP AGAIN TO DO SOMETHING THE RADAR CTLR INITIATED A DSCNT TO ACFT #1 AS WELL AS DIVERGING TURNS TO BOTH ACFT. I BELIEVE TIME ON POS OVER THE LAST 4 MONTHS HAS BEEN AT AN ALL TIME HIGH. CTLRS CONSISTENTLY WORK UP TO 2 HRS AND 10 MINS IN ONE SESSION; SOMETIMES WORKING A BUSY PUSH AND SOON FALL INTO THE NEXT PUSH. AFTER SAYING AND HEARING SO MANY NUMBERS; AND CONCENTRATING SO INTENTLY FOR LONG PERIODS OF TIME; IT IS EASY TO HEAR WHAT YOU WANT TO HEAR; AND THINK YOU DID WHAT YOU WANTED TO DO. DURING THE INCIDENT TFC WAS SLOW; UNFORTUNATELY A PERIOD WHEN THE CTLR CAN TAKE A 'BREATHER'; BUT STILL MUST MAINTAIN A SHARP CONCENTRATION LEVEL.