MLT X CLB THROUGH OCCUPIED ALT HAD NMAC WITH SMA Y. POSSIBLE LTSS SYS ERROR. SEE AND AVOID CONCEPT.

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THIS NEAR COLLISION OCCURRED IMMEDIATELY PRIOR TO A POS RELIEF BRIEFING. I BEGAN MONITORING THE CTLR; WHO WAS TO BE RELIEVED; WORKING THE POS. WITHIN A MIN; I INDICATED TO THE CTLR THAT I WAS READY TO BEGIN THE RELIEF BRIEFING. IT WAS THEN THAT THE 'CONFLICT ALERT' ALARMED. I SAW MLT X CLBING INTO AN ENRTE ACFT. THE OTHER ACFT WAS SMA Y. BOTH ACFT WERE ON INST FLT PLANS; AND BOTH ACFT WERE COMPLYING WITH THEIR CLRNCS. SMA Y WAS FLYING NWESTERLY ALONG VICTOR 171 AT 8000 FT MSL. MLT X WAS FLYING SEASTERLY TOWARD THE LOUISVILLE VORTAC AND WAS CLBING TO AN INTERMEDIATE ASSIGNED ALT OF 10000 FT MSL. THE NEAR COLLISION OCCURRED WHEN MLT X WAS IN FRONT OF SMA Y ON AN OPPOSITE DIRECTION COURSE; AND CLBED THROUGH THE SAME ALT OF SMA Y. I BECAME AWARE OF THE LOSS OF APPROVED SEPARATION WHEN THE 2 ACFT WERE BTWN 1 AND 2 MI APART. AT THAT TIME; MLT X ALT READOUT; MODE C; INDICATED 7000 MSL; AND SMA Y ALT INDICATED 7900. AFTER READING THE FLT PROGRESS STRIP OF MLT X; I SAW AN INSCRIPTION OF THE FLT PROGRESS STRIP INDICATING MLT X WAS ASSIGNED 10000 FT MSL. AS MLT X AND THE SMA Y DREW NEARER; THE ALT READOUT OF MLT X INDICATED CLBING THROUGH 7200 FT MSL. AT THIS TIME; WHEN THE 2 ACFT WERE FACE TO FACE AND ABOUT 1 MI APART; I POINTED TO THE 2 ACFT AND SAID; IS MLT X CLBING TO 10000? WHEN THE CTLR DID NOT RESPOND I SAID AGAIN; IS MLT X CLBING TO 10000? THE CTLR DID NOT RESPOND AND THE 2 ACFT'S RADAR TARGETS EMERGED WITH NO DISTINGUISHABLE HORIZ SEPARATION AND AN ESTIMATED VERT SEPARATION OF LESS THAN 500 FT. WHEN THE MERGING ENDED; MLT X ALT READOUT INDICATED CLBING THROUGH 8000 FT MSL. THE CTLR WORKING THE POS TOLD ME; AFTER THE NEAR COLLISION; THE ACFT WAS OPERATING UNDER VFR AND THAT HE DID NOT DESIRE TO BE RELIEVED FROM THE POS.