MULTIPLE RWY OP PARALLEL RWYS; SAME RWY APCH; ACR Y HAD CONFLICT WITH ACR X. VISUAL APCH VISUAL SEPARATION. SEE AND AVOID CONCEPT.

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ACR Y CLRED FOR A VISUAL APCH TO RWY 26R AT DEN SUBJECT TO MAINTAINING VISUAL SEPARATION WITH ACR X COMMUTER ACFT ON FINAL TO RWY 25. ACR X WAS OUR ALT AND AHEAD OF US ON STRAIGHT-IN FINAL TO RWY 25. AS WE WERE TRYING TO CONTACT THE TWR (LOTS OF RADIO TFC) ACR X SLID TO THE L DIRECTLY IN FRONT OF US AND LINED UP WITH RWY 26R. THE TWR EVIDENTLY FORGOT ABOUT US; AND HAD ACR X SIDESTEP TO RWY 26R TO MAKE ROOM FOR ANOTHER COMMUTER THAT WAS ON FINAL FOR RWY 25 BEHIND THE FIRST ONE. TWR THEN NOTICED THE PROB AND HAD THE SECOND COMMUTER BREAK OFF THE APCH AND HAD ACR X RETURN TO RWY 25 FINAL. LNDG WAS NORMAL ON RWY 26R. ACR X LANDED RWY 25. SUPPLEMENTAL INFO FROM ACN 229403: ON APCH TO DEN; ACR X WAS ASKED IF WE COULD ACCEPT RWY 25 FOR LNDG. WE ACKNOWLEDGED YES AND WERE CLRED FOR A VISUAL APCH TO RWY 25. WE CONTACTED DEN TWR ABEAM ALTUR LO OM. WE WERE CLRED TO LAND RWY 25 -- CAUTION FOR WAKE TURB OF WDB ON FINAL TO RWY 26L. AT APPROX 6300 FT MSL (1000 FT AGL) A COMPANY FLT CHKED IN TO DEN TWR FOR RWY 25 LNDG. AFTER CLRING HIM TO LAND; OUR FLT WAS TOLD TO CHANGE TO RWY 26R -- CLRED TO LAND. I MANEUVERED OVER TO RWY 26R WHEN ANOTHER ACR Y ASKED THE TWR WHAT WAS GOING ON. HE ASKED IN AN URGENT MANNER. DEN TWR TOLD ACR Y HE WAS CLRED TO LAND RWY 26R. HE TOLD US TO CHANGE BACK TO RWY 25 -- CLRED TO LAND. THE CTLR THEN TOLD THE COMPANY FLT TO GAR PROVIDING HIM WITH VECTORS. THIS ALL OCCURRED AT APPROX 500 FT AGL. AS SOON AS I MANEUVERED BACK TO RWY 25; ACR Y PASSED OFF OUR L WING.