ACR X RESPONSE TO WRONG CALL SIGN UNAUTH DSCNT THROUGH OCCUPIED ALT HAD LTSS FROM ACR Y. PLTDEV. SYS ERROR.

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I WAS WORKING THE FFO RADAR SECTOR. THE AIRPLANES INVOLVED WERE: ACR X T/757/G SNA/PIT ACR Y T/727/A LGA/STL ACR ZT/B757/G SAN/PIT. X WAS ISSUED A CLRNC TO DSND FROM FL410 TO FL370 ACR Y WAS WBOUND ON J-80. AT FL350 X WAS THEN ISSUED A CLRNC TO TURN 20 DEGS L FOR FURTHER DSCNT. 20 SECONDS LATER Z WAS TOLD TO DSND FROM FL410 TO FL330. 6 SECONDS LATER X ACCEPTED AND READBACK THE CLRNC FOR Z. X; WHO WAS TCASII EQUIPPED DID NOT BEGIN HIS DSCNT UNTIL 40 SECONDS HAD ELAPSED. HE DID NOT QUESTION THE CLRNC EVEN THOUGH HE OBVIOUSLY KNEW ACR Y WAS TFC. WHEN THE SIT WAS NOTICED Z WAS QUESTIONED ABOUT HIS CLRNC. THE REPLY FROM Z WAS; 'SOMEBODY ELSE TOOK THAT CLRNC.' THIS WAS 1 MIN AND 20 SECONDS AFTER THE CLRNC WAS ISSUED. AT THIS TIME ADEP HAD ALREADY ACTIVATED. SUPPLEMENTAL INFO FROM ACN 285002: I HAD BEEN WORKING THE FFO MANUAL CTLR FOR 3 MINS PRIOR TO ERROR. ACFT #1 -- ACR Y T/B727/A VIA LGA J-80 STL AT FL350. ACFT #2 -- ACR X T/B757/G VIA SNA DIRECT APE PIT DSNDING FL330. X WAS ISSUED A VECTOR 20 DEGS L FOR THE DSCNT INTO PIT. 20 SECONDS LATER; COMPANY FLT XXX (T/B757/G; ALSO TO PIT) WAS CLRED TO DSND AND MAINTAIN FL330. X MISTAKENLY TOOK CLRNC AND HIS READBACK WAS PARTIALLY COVERED BY OTHER ACFT AND ADJACENT SECTOR CALL ON HDOF LINE. CONFLICT ALERT AND ADEP ACTIVATED 1 MIN AND 20 SECONDS LATER AS X PASSED 2 MI R AND 1200 FT BELOW Y. UPON QUESTIONING Z; IF HE HAD RECEIVED/ACKNOWLEDGED DSCNT TO FL330 HE RESPONDED; 'I THINK SOMEONE ELSE TOOK THAT CLRNC.' MY CONCERN IS WHY DID Z NOT QUESTION CLRNC IF HE THOUGHT IT WAS FOR HIM.