AN ARTCC RADAR CTLR RPTS THAT AN ACR MD80 FLC DSNDED BELOW ITS ASSIGNED ALT IN A HOLDING PATTERN AND CONFLICTED WITH ANOTHER ACR THAT WAS ALREADY IN THE HOLDING PATTERN. THE CTLR; APPARENTLY; MISSED AN INCORRECT READBACK BY THE MD80 FLC. THE FREQ WAS RPTED TO BE CONGESTED.

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I WAS CLRING ACFT INTO THE HOLDING PATTERN AT THE BDF VOR. THE FIRST ACFT CLRED IN WAS GIVEN A CLRNC TO DSND AND MAINTAIN FL240. THE SECOND ACFT CLRED IN WAS TOLD TO CROSS BDF AT FL250. THE PLT OF THE SECOND ACFT READ BACK DSND AND MAINTAIN FL250 WHICH I CORRECTED BY REPEATING THAT THEY COULD CROSS BDF AT FL250. THE PLT READ THIS BACK CORRECTLY. THE FIRST ACR WAS ALREADY ESTABLISHED IN THE HOLD WHEN I ASKED THE SECOND ACR TO BEGIN HIS DSCNT AND HE RPTED THAT HE WAS LEAVING FL290 TO WHICH I IMMEDIATELY RESPONDED 'ROGER.' IN LISTENING TO THE TAPE AFTERWARD; IT SOUNDED LIKE MY ROGER COVERED PART OF HIS XMISSION; BUT AT THE TIME I DID NOT HEAR THAT. THE FIRST ACR WAS LEVEL AT FL240 WHEN THE SECOND ACR RPTED ENTERING THE HOLD AT WHICH TIME I LOOKED AT THE DATA BLOCK AND SAW THAT IT SAID FL240. I ASKED THE SECOND ACR TO SAY ALT AND HE RPTED LEVEL FL240 SO I CLBED HIM TO FL250 AND TURNED HIM TO A 360 DEG HDG. AT THIS TIME BOTH PLTS RESPONDED TO TCASII RA'S AND FIRST ACR CLBED TO FL250 AND THE SECOND ACR DROPPED DOWN TO FL236 BEFORE I CLBED HIM BACK UP TO FL240. SUPPLEMENTAL INFO FROM ACN 365300: WHILE HOLDING AT BDF VOR ENRTE TO ORD; WE HAD BEEN CLRED TO AND WERE LEVEL AT FL240. WE RECEIVED A TCASII TA FOLLOWED BY A CLB RA. WE RESPONDED IMMEDIATELY AND CLBED TO FL247 AND NOTIFIED CTR. ANOTHER ACR ALSO RESPONDED TO AN RA WITH A DSCNT. WE WERE RECLRED BY CTR TO FL250. WE NEVER HAD VISUAL CONTACT WITH THE OTHER ACR. ONCE ON THE GND; WE TELEPHONED CTR AND THE ATC SUPVR TOLD US THAT THE OTHER ACR HAD BEEN CLRED TO FL250; BUT READ BACK FL240. THE CTLR DID NOT CATCH THE ERROR AND THE OTHER ACR DSNDED TO FL240.