ZID CTLR REALIZES SPD OVERTAKE SIT TOO LATE TO MAINTAIN LEGAL SEPARATION.

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FIRST ACFT SLOWED TO 250 KTS. SECOND ACFT WAS OVERTAKING. SECOND ACFT WAS TURNED TO AVOID CONFLICT WITH FIRST ACFT IN DSCNT. HDGS SHOULD HAVE BEEN ASSIGNED EARLIER. FIRST ACFT'S SPD SHOULD HAVE BEEN DELETED. SUPPLEMENTAL INFO FROM ACN 518054: I WAS ACTING CIC; MONITORING A BUSY SECTOR WITH A HDOF POS BEING UTILIZED. I THEN NOTICED ANOTHER SECTOR GETTING BUSY AND ASSIGNED A RADAR ASSOCIATE POS TO ASSIST A RADAR ONLY SECTOR. 2 MINS AFTER THE ASSOCIATE PLUGGED IN THE ERROR OCCURRED. THE RADAR ASSOCIATE WAS ON A COORD LINE WHEN THE RADAR CTLR DSNDED THE SECOND ACFT THROUGH THE ALT OF THE FIRST ACFT. IN MY OPINION THERE ARE 3 CAUSAL FACTORS IN THIS ERROR. FIRST AND FOREMOST; THE INEXPERIENCE AND INADEQUATE ABILITY OF THE RADAR CTLR. HE WAS CERTIFIED (AND SOME WOULD QUESTION WHY) APPROX 3 MONTHS AGO. SECONDLY; HE WAS UNAWARE OF A SPD DIFFERENCE (FASTER ACFT IN BACK) EVEN THOUGH THE SLOWER SPD ON THE LEAD ACFT HAD BEEN COORDINATED. AND LASTLY; HE TURNED THE TRAILING ACFT S WHEN HE PROBABLY SHOULD HAVE TURNED HIM N.