IAD CTLR DESCRIBED AMASS ALERT RECEIVED FOR ACFT ON LNDG FLARE; NO GAR ISSUED; KNOWN XPONDER ANOMALY LISTED AS PROBABLE CAUSE.

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ACR X OVER THE THRESHOLD FOR RWY 19R AT IAD CAUSED AN AMASS ALERT DUE TO A DUPLICATE BEACON THAT THE ARTS COMPUTER PICKED UP BEACON CODE XXX AMASS SAFETY LOGIC SOFTWARE ISSUED A GAR THINKING THAT THERE WERE 2 ACFT ATTEMPTING TO LAND AT THE SAME TIME. THE ACFT WAS BEYOND THE THRESHOLD AND IN THE FLARE AND WAS NOT SENT AROUND. PLT CONFIRMED THAT HE WAS SQUAWKING THE ASSIGNED CODE OF XXX. OUR OWN TECHNICAL OPS PERSONNEL SAID THERE WAS ANOTHER TARGET AT FL300 THAT MAY HAVE CAUSED A GHOST TARGET THAT WAS CAPTURED BY THE INTERFACE BTWN ASR AND SURFACE RADAR. QUERY TO FAA SURFACE RADAR SYS TEAM REVEALED THAT A CERTAIN XPONDER; THE ROCKWELL-COLLINS TPR-901; WILL CAUSE A DUPLICATE BEACON OF XXX. THE COMPANY KNOWS THIS AND A MODIFICATION TO FIX THE PROB IS AVAILABLE. IF THERE ARE ACFT OUT THERE FLYING AROUND WITH THIS PARTICULAR XPONDER; A SAFE SITUATION CAN BE MADE UNSAFE IF A CTLR REACTS INCORRECTLY TO WHAT OUR ENGINEERS CALL A NUISANCE ALERT (VICE FALSE ALERT). AN AIRWORTHINESS INSPECTOR HAS THE INFO AND HOPEFULLY WILL ATTEMPT TO CONTACT THE COMPANY TO SEE IF THEY ARE OPERATING WITH THAT MODEL XPONDER.