APCH CTLR MISIDENTED ACFT (ATX X AND SMA). ATX X HAD LTSS FROM LTT Y. SYS ERROR.

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SECOND ACFT WAS ATX X CARGO ACFT INBOUND TO GEG; LTT Y OUTBOUND. ATX X TAGGED UP ON RADAR AS A VFR SMA FROM AN OLD FLT PLAN REMAINING IN THE ARTS II A COMPUTER. CTR CTLR INDICATED TO GEG E RADAR CTLR THAT ATX X WAS 4 MI E OF SMA TAG. I WAS WORKING LTT Y OPPOSITE DIRECTION ON GEG W RADAR. AS SMA TAG CONTINUED NEBOUND; APCHING LTT Y I SUSPECTED SOMETHING WAS WRONG AS IT CONTINUED INBOUND ON DEP CTRLINE AND SHOWED A GNDSPD TOO HIGH FOR AN SMA. I WAS BUSY SIMULTANEOUSLY WITH A MIL OP; BUT INSTRUCTED THE LTT Y TO DSND TO 5000 IMMEDIATELY. THE E CTLR SIMULTANEOUSLY REALIZED THE SMA TAG WAS ACTUALLY THE ATX X AND TURNED IT EBOUND. BOTH ACFT SAW THE OTHER VISUALLY. SEATTLE CTR CTLR SHOULD NEVER HAVE ISSUED THE POS OF THE UNIDENTED ATX X AS E OF A NAMED TARGET HE DID NOT HAVE DISPLAYED ON HIS SCOPE (THE SMA). WHEN THE HOST COMPUTER DROPS A FLT PLAN (AS IT HAD THE SMA); IT IS IMPERATIVE THAT IT ALSO DROP IT FROM ANY ASSOCIATED ARTS SYS SO THAT AN ACFT LATER ASSIGNED THE SAME CODE CANNOT TAG TO A PREVIOUS UNKNOWN ACFT FLT PLAN. 'IDENT' AND CODE-READOUT CHKS BY CTLRS ON ANY AND ALL QUESTIONABLE TARGETS SHOULD BE PERFORMED RELIGIOUSLY. SUPPLEMENTAL INFO FROM ACN 221891. CTR WAS UNABLE TO HDOF ATX X. I TOLD THEM I WAS BUSY AND WOULD CALL THEM BACK. CTR ASKED IF THEY COULD KEEP HIM COMING AND I SAID YES. THEN I CALLED BACK AND ASKED WHERE ATX X POS WAS REF A VFR DATA BLOCK; THEY SAID HE WAS 4 MI E OF HIM THEN THEY ASKED IF THEY COULD TERMINATE AND SHIP HIM; I AGREED. I TURNED UP MY PRIMARY RADAR TO LOOK FOR A PRIMARY AND TOLD W RADAR I HAD AN ATX X TO THE S AT 60 AND DID NOT KNOW WHERE HE WAS. ATX X CHKED ON AND I ASKED HIM HIS ALT DME AND RADIAL; WHICH HE GAVE. HIS POS WAS WHERE THE VFR SMA DATA BLOCK WAS. I QUICK LOOKED HIS BEACON CODE AND IT WAS THE SAME AS ATX X CODE. THEN RECOGNIZING THAT THE VFR DATA BLOCK WAS REALLY ATX X I GAVE HIM TFC AND TURNED HIM. HE TOLD ME HE HAD THE TFC IN SIGHT. I TOLD HIM TO MAINTAIN VISUAL SEPARATION. THE TFC FOR ATX X WAS BEING WORKED BY THE W CTLR AND THERE WAS A LOSS OF SEPARATION. IT APPEARS THAT THE FLT PLAN ON THE VFR SMA WAS DROP FROM THE CTRS COMPUTER BUT NOT FROM OURS SO ATX X TAGGED UP AS SMA.