CPR SMT TOOK THE DESCENT CLRNC MEANT FOR CPR LTT AND THE CTLR MISSED THE ERROR IN READBACK. LESS THAN STANDARD SEPARATION BETWEEN THESE TWO ACFT.

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I WAS ASSIGNED SECTOR 26 ON A XA00-XI00 SHIFT. AFTER TAKING A BREAK I RELIEVED THE SECTOR 27 RADAR CTLR AT ABOUT XC40 AM. LTT X (ABC AB) WAS ENRTE TO DCA VIA SEG BAL DCA AT FL180. SMT Y (XYZ AB) WAS ENTRE TO PHL VIA HAR LRP V210 BUCKS PHL; DSNDING TO 110. LGT Z WAS ALSO ENRTE TO PHL. I DECIDED TO LET LGT Z BE FIRST OVER LRP. TO ACCOMPLISH THIS I STOPPED SMT Y (XYZ AB'S) DSCNT AT FL190 AND TURNED HIM TO A 070 DEG HEADING TO GO BEHIND LGT Z. I STOPPED SMT Y (XYZ AB) AT FL190 TO GO OVER LTT X (ABC AB) SINCE I HAD TURNED SMT Y (XYZ AB) TOWARDS LTT X (ABC AB). I THEN CLRED LTT X (ABC AB) TO DSND TO 130; AS REQUIRED BY AREA POLICY. HOWEVER SMT Y (XYZ AB) BECAUSE OF SIMILARITY OF CALL SIGNS MISTAKENLY TOOK THE DSCNT CLRNC AND I MISSED HIS READBACK. WHEN I NOTICED SMT Y (XYZ AB'S) MODE C READOUT AT FL187; I ASKED HIM TO VERIFY LEVEL AT FL190. THE PLT REPLIED; 'NEGATIVE; I HAVE BEEN CLRED TO DSND TO 130.' I THEN REPLIED 'NEGATIVE; YOU WERE CLRED TO MAINTAIN FL190.' SMT Y (XYZ AB) THEN; AFTER GOING AS LOW AS FL184; CLBED BACK TO FL190; HOWEVER; HE WAS ALREADY WITHIN 5 MI OF LTT X. I BELIEVE LTT X WAS HAVING PROBLEMS WITH HIS RADIO BECAUSE HE DID NOT RESPOND TO MY NEXT THREE CONTROL INSTRUCTIONS; THE FIRST OF WHICH WAS TO DSND. HUMAN PERFORMANCE CONSIDERATIONS INCLUDE SMT Y (XYZ AB) PLT PERCEPTION OF EXPECTING LOWER ALT AND MY OWN BELIEF IN HEARING WHAT I PERCEIVED AND NOT WHAT WAS REALLY SAID. ALSO MY WORKING TO LET LGT Z BE FIRST INSTEAD OF SECOND WHICH WOULD HAVE BEEN EASIER.