B737 LNDG DEN OBSERVED PARALLEL TFC LINING UP WITH WRONG RWY AND EXECUTES GAR.

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AFTER A GAR DUE TO WINDSHEAR; WE WERE BEING VECTORED BACK TO INTERCEPT A VISUAL APCH RWY 16L. ARPT WAS CALLED IN SIGHT AND WE WERE CLRED FOR VISUAL RWY 16L. TFC WAS ALSO IN SIGHT AND CLRED FOR VISUAL RWY 16R (ACR Y). ACR Y APPEARED TO BE LINING UP FOR RWY 16L BELOW US. WE STAYED WELL L OF RWY 16L AND CONFIRMED TFC WAS FOR RWY 16R WITH APCH. THE FO CONFIRMED THE TFC MOVING BELOW US AND WE FLEW L OF RWY 16L; DISCONTINUED THE APCH; COORDINATED WITH ATC TO BREAK OFF THE APCH; DECLARED MINIMUM FUEL. SUBSEQUENT APCH TO RWY 16L WAS UNEVENTFUL. WE ALSO CONFIRMED PROPER VISUAL ALIGNMENT; NAVAIDS AND COURSES; AS WELL AS PROPER VISUAL CLUES. CONTINUING ON THE APCH WOULD HAVE RESULTED IN DEFINITE LOSS OF SEPARATION. I CALLED TWR AND APCH AFTER EVENT FOR THEM TO FOLLOW UP ON CONFLICT. SUPPLEMENTAL INFO FROM ACN 615941: I LOOKED OFF THE R SIDE OF THE ACFT AND ABOUT 2000 FT BELOW US WAS AN A319/320. HE APPEARED TO BE ON THE FINAL APCH COURSE FOR RWY 16L. WE ASKED APCH TO VERIFY THAT THE OTHER ACFT WAS ON THE RWY 16R APCH COURSE. WE CALLED THE TWR AND THEY WERE NOT AWARE OF ANY ACFT THAT LANDED ON THE WRONG RWY. WE ALSO CALLED APCH AND ASKED THE SAME QUESTION. THEY REPLIED THAT THEY WOULD REVIEW THE TAPES. I WOULD BET MONEY THAT WE WOULD HAVE HIT THEM HAD WE CONTINUED OUR APCH TO RWY 16L.