B767 LNDG DFW EXPERIENCES POTENTIAL CONFLICT WITH DEP ACFT.

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SHORT FINAL; RWY 17R; APCH. ARRIVING DFW AFTER 10 HR FLT. SUN NOT UP YET. ARPT IN MORNING TWILIGHT. WE WERE CLRED TO LAND ON DFW RWY 17R. AT 400-500 FT AGL; APPROX 1.5 MI OUT; I HEARD TWR CLR AN ACR Y FLT 'INTO POS AND HOLD; ARR TFC IS ON RWY 17C.' THE ACR Y FLT ACKNOWLEDGED THE TFC; US; AND STARTED TAXI ONTO THE RWY. I WAS MAKING LNDG; BUT SAW HIS LIGHTS MOVE FORWARD IN FRONT OF US AND IMMEDIATELY CALLED TWR ADVISING; 'TWR; ACR X HAVE YOU CLRED AN AIRLINE ONTO OUR RWY. WE ARE LNDG RWY 17R.' RELIEF PLT LATER SAID THAT HIS HEAD WAS DOWN; LISTENING TO MAINT FREQ. FO SAW THE ACFT BUT ONLY PARTIALLY HEARD HIS CLRNC; THE PART ABOUT US SUPPOSEDLY LNDG ON RWY 17C. TWR CAME BACK; CANCELING THE POS AND HOLD CLRNC; ADVISING ACR Y TO 'HOLD SHORT.' SINCE THE FORWARD MOVEMENT OF THE ACR Y WAS EVIDENT; BUT WE COULD NOT DISTINGUISH THE 'HOLD SHORT' LINE IN THE DARKNESS; I THEN ASKED ACR Y IF HE WAS CLR OF THE RWY. HE SAID HE WAS AND WE COMPLETED THE APCH AND LNDG. AFTER WE TOUCHED DOWN; WE HEARD TWR AGAIN CLR ACR Y ONTO RWY 17R. HE MADE NO APOLOGIES NOR EXPLANATIONS AS TO WHY HIS ERROR OCCURRED. ACR Y TOOK OFF AS WE CLRED THE RWY. DFW E TWR SUPVR WAS CALLED AT XA00 APPROX 1 HR 50 MINS AFTER THE INCIDENT AND PROVIDED WITH PERTINENT FACTS. HE SAID HE WOULD REVIEW ATC TAPES AND START AN INTERNAL QUALITY ASSURANCE PROCESS. HE WAS VERY HELPFUL.