MIL X ERRONEOUS DSCNT TO OCCUPIED ALT HAD LTSS FROM ACR Y. SYS ERROR.

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MLT X ISSUED CLRNC TO CROSS PUTTZ INTXN AT FL270. DID NOT ACKNOWLEDGE. REISSUED CLRNC AND MISTAKENLY GAVE CROSS PUTTZ AT FL240. ACFT ACKNOWLEDGED. THE DATA BLOCK SHOWED ACFT DSNDING TO FL270 WITH ACR Y JUST IN FRONT; LEVEL AT FL240. BOTH ACFT CHANGED TO NEXT CTLR AFTER HDOF. MLT X CHKED ON OUT OF FL290 DSNDING TO FL240. MISTAKE WAS NOT CAUGHT AND SHORTLY THEREAFTER LOSS OF SEPARATION OCCURRED. SUPPLEMENTAL INFO FROM ACN 238114: MIL X CLRED TO CROSS THE PUTTZ INTXN ON THE FINKS 5 ARR INTO WASHINGTON; DC; AT FL240. CLRNC WAS RECEIVED AT APPROX 60 DME BKW. (PUTTZ IS THE 072/95.) UPON ACKNOWLEDGING THE CTR CLRNC; THE CREW SET AND CONFIRMED THE ALT IN THE ALERTER AND BEGAN THE DSCNT. FREQ CHANGE WAS GIVEN TO DULLES APCH. THE CREW CHKED IN '...DSNDING TO FL240' AT WHICH TIME WE WERE ASKED TO MAINTAIN FL250; WE WERE ALREADY THROUGH FL250 AND IN FACT SLOWING OUR VVI TO APPROX 200-300 FPM AND ACTUALLY AT FL247; WE LEVELED AT FL244 AND CLBED BACK TO FL250. WE SPOTTED AN ACR Y AT OUR 1-2 O'CLOCK POS CONVERGING. THE CTLR IMMEDIATELY GAVE ACR Y A CLRNC TO DSND. WE WERE ASKED TO CONTACT DULLES FOR POSSIBLE ACTION. CONTACT WAS MADE WITH DULLES AND WE WERE TOLD NO FURTHER INFO WAS REQUIRED OF THE CREW THAT 'CTLR ERROR' WAS NOTED.