TRACON APCH CTLR BECOMES AWARE; AS THE RESULT OF AN ACFT COMMENT; THAT 2 ACFT ARE TAGGED AND BEING RADAR TRACKED WITH THE SAME CALL SIGN.

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AIRPLANE #1 (ACFT X) ON MUSTANG 5 SID; RESTR TO 10000 FT. AIRPLANE #2 (ACFT Y) ARRIVING FROM SE VIA J92; DSNDING TO 11000 FT; HDG 340 DEGS AFTER FMG. ACFT #1 WAS TURNED TO 330 DEGS TO JOIN J92 NWBOUND. ACFT #2 QUESTIONED INSTRUCTION; I REPEATED IT. AT THAT POINT; PLT SAID 'I THINK YOU HAVE 2 ACR XXX ON THIS FREQ.' I THEN REALIZED THAT BOTH THE DEP AND ARR DATA BLOCKS WERE IDENTICAL. IN 24 YRS OF ATC; I HAVE NEVER HAD 2 IDENTICAL CALL SIGNS IN MY SECTOR. I WAS ALWAYS TOLD IT WAS NOT POSSIBLE; OR THAT 'DUPE IDENTS' WOULD SHOW UP SOMEHOW. OUR NEW ARTS IIE SYS AND THE NAS SYS PERMITTED THIS TO HAPPEN. I DID GET A 'DEP MESSAGE' IN THE DATA BLOCK OF ACFT #1; AND TWR DID A DEP MESSAGE VIA FDIO; RESULTING IN 'DUPLICATE FLT ACTIVE;' WHICH IS THE SAME RESPONSE RECEIVED WHEN THE DATA BLOCK IS SLEW ACKNOWLEDGED BY DEP. I CALLED FOR ASSISTANCE AND ONE ACFT WAS DESIGNATED ACR XXX-A. THE SURPRISE UPON REALIZING THE PROB WAS PROFOUND. THOUGH THE CLOSEST POINT OF APCH WAS NOT A FACTOR; THIS SIT WAS POTENTIALLY VERY SERIOUS. MY WORKLOAD AT THE TIME WAS MEDIUM (6 OR 7 ON FREQ). I THINK RECENT COMPUTER CHANGES HAVE LOST SOME OF THE BUILT-IN SAFEGUARDS PREVIOUSLY INSTALLED. ALL CTLRS SHOULD BE WARNED OF THIS DANGEROUS POSSIBILITY. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR VERIFIED THE INCIDENT TOOK PLACE. HIS CONCERN IS THAT THE NEW ARTS IIE SYS HAS LOST A VERY IMPORTANT SAFEGUARD THAT WAS IN PLACE PRIOR TO THIS NEW BUILD. THE RPTR WAS IN THE PROCESS OF FURTHER FACILITY INVESTIGATION WITH MGMNT TO ATTEMPT RESOLUTION.