LCL CTLR AT HOU ISSUED A LNDG CLRNC TO AN MD80 FOR RWY 30L WITH A B737 LNDG ON THE OPPOSITE END.

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AT THE TIME; HOU WAS LNDG MULTIPLE RWYS WITH CLR SKIES AND CALM WINDS. WE WERE FLYING A VISUAL APCH TO RWY 30L AND ANOTHER CARRIER; A VISUAL TO RWY 12R. AS WE TURNED FINAL AT THE MARKER; THE OTHER ACFT WAS INSIDE 2 MI AS RPTED BY TWR. THE FO WAS FLYING THE APCH NORMALLY AND BOTH OF US TRYING TO VISUALLY ACQUIRE THE OTHER ACFT. AT 2 MI ON FINAL; SUDDENLY THE OTHER ACFT CALLED OUT URGENTLY; 'ACFT ON RWY; GO AROUND ON FINAL.' AT THE SAME TIME; EVERY EXTERNAL LIGHT ON THAT ACFT ILLUMINATED. TWR INSTRUCTED THE OTHER ACFT TO EXIT AT THE HIGH SPD OR EXPEDITE TO THE END AND FOR US TO CONTINUE (WE ALREADY HAD LNDG CLRNC). WE HAD ALREADY DECIDED TO INITIATE A GAR AND THUS CONTINUED WITH THAT PLAN OF ACTION. WE ACCOMPLISHED A BOX PATTERN TO AN UNEVENTFUL LNDG. CONTRIBUTING FACTORS: 1) I DO NOT BELIEVE THE OTHER ACFT WAS AWARE OF THE SIT; IE; WHEN HE SAW US ON SHORT FINAL; IT WAS HIS FIRST REALIZATION THAT ACFT WERE LNDG OPPOSITE RWYS IN CLOSE PROX. 2) I DO NOT BELIEVE HE HAD ALL OF HIS EXTERNAL LIGHTS ON UNTIL HE SAW A CONFLICT. 3) AT A 2 MI FINAL; I SHOULD HAVE BEEN PROMPTING TWR FOR MORE INFO CONCERNING THE OTHER ACFT. SUPPLEMENTAL INFO FROM ACN 558391: WHILE FLYING INTO HOU HOBBY LATE AT NIGHT; APCH WAS LETTING EACH AIRPLANE PICK THEIR OWN RWY. WE SELECTED RWY 30L; AS DID THE AIRPLANE BEHIND US. ACFT Y PICKED RWY 12R. HE WAS WELL AHEAD AND WAS NOT A FACTOR (OR SO WE THOUGHT). WE FLEW A VISUAL PATTERN AND WERE TRACKING THE LOC INTO RWY 30L. WHILE ON FINAL; THE RWY LOOKED CLR. WHEN WE WERE ABOUT 500-700 FT AGL; WE SIMULTANEOUSLY SAW BRIGHT LIGHTS ON THE RWY AND HEARD A VERY EXCITED PLT TELLING; 'ACFT ON FINAL GO AROUND;' WHICH WE DID. ABOUT 10-15 SECONDS INTO THE GAR; TWR ASSURED US THE RWY WAS CLR AND WE WERE CLRED TO LAND. WE CONTINUED GAR AND MADE ANOTHER PATTERN TO UNEVENTFUL LNDG.