CTLR DSNDED ACR X TO OCCUPIED ALT HAD LTSS FROM ACR Y. SYS ERROR. ACR X TCASII TA RA CLB. ACR Y TCASII RA DSND. EVASIVE ACTION TAKEN.

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ACR Y ON FREQ NBOUND FL260 APPROX 40 MI NW LOU. ACR X CHKED ON FREQ APPROX 55 MI NW LOU DSNDING FL280; EBOUND. I INTENDED TO CLR ACR X TO FL270 BUT INADVERTENTLY CLRED ACR X TO FL260. TFC WAS ISSUED TO ACR X WITH NO REPLY. I AGAIN ISSUED TFC TO ACR X WITH A CLRNC TO MAINTAIN FL270 WITH NO REPLY AGAIN. ACR Y ADVISED THEY WERE DSNDING DUE TO AN RA ON TCASII. I CLRED ACR X TO CLB AND MAINTAIN FL270 AND AGAIN NO REPLY; HOWEVER; ACR X EXECUTED AN IMMEDIATE CLB TO FL270. AT THIS POINT; SEPARATION IS LOST. AT THE TIME OF THIS INCIDENT; I BELIEVED THIS TO BE A MODE 'C' SWAP BECAUSE THE TARGETS MERGED AND ACR X RAPID MODE C CHANGE WITH NO REPLY; AS WELL AS TCASII RA. WHY DIDN'T ACR X QUESTION HIS CLRNC TO FL260 AFTER BEING TOLD HIS TFC IS AT FL260. WHY DIDN'T ACR X ANSWER THE 3 CLRNC THAT COULD HAVE POTENTIALLY AVOIDED THIS PROB. SUPPLEMENTAL INFO FROM ACN 235548: THE RADAR CTLR ISSUED TFC TO ACR X AGAIN SAYING THE TFC WAS 'AT FL260 MAINTAIN FL270.' AGAIN; THE ACR X DID NOT RESPOND. WHEN CONFLICT ALERT ACTIVATED; THE RADAR CTLR TOLD ACR X TO CLB AND MAINTAIN FL270. STILL; ACR X DID NOT RESPOND. SUPPLEMENTAL INFO FROM ACN 234545: ACR X ON ZID CLRED FROM FL280 TO FL260; COPLT READ BACK CLRNC TO DSND TO FL260. WHILE DSNDING THROUGH FL270; A TCASII WARNING TA THEN IMMEDIATELY TO RA WAS PRESENTED AT ABOUT THE SAME TIME CTLR TOLD US TO CLB TO FL270. WE IMMEDIATELY INITIATED CLB TO FL270. CONFLICT WAS WITH ACR Y WHO WAS AT FL260. THIS ACFT WAS PASSING AT APPROX R ANGLES TO OUR FLT PATH. THEY ALSO RECEIVED A TCASII WARNING AND DSNDED. AFTER CONFLICT WAS RESOLVED; WE ASKED THE CTLR WHAT THE PROB HAD BEEN. HE RESPONDED; 'NO PROB; NO BIG DEAL; SWITCH TO FREQ.'