B777 RPTS POSSIBLE CONFLICT AT DIA WITH 2 ARR ACFT.

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ON ILS TO RWY 1L AT IAD. WX 1200 FT BROKEN VISIBILITY 8 NM. CROWDED ILS PATTERN. APCH CTL VERY BUSY. AFTER VISUALLY ACQUIRING THE RWY; I SAW WHAT APPEARED TO BE A REGIONAL JET ON THE RWY. I REMARKED TO MY FO THAT THE JET WAS PROBABLY IN POS FOR TKOF. TWR ISSUED US LNDG CLRNC. PASSING 500 FT AGL; WE REALIZED THAT THE JET HAD ACTUALLY LANDED PRIOR TO US AND WAS STILL ON THE RWY. IT APPEARED THE JET HAD TOUCHED DOWN 'ON THE NUMBERS' AND ROLLED OUT LEISURELY ON THE RWY. I SAID TO MY FO; 'GET READY FOR A GAR' BECAUSE HE HADN'T YET CLRED RWY 1L. TWR CTLR APPARENTLY DID NOT NOTICE WHAT WAS TRANSPIRING. AT 50-75 FT AGL; TWR SAID 'ACR X; GO AROUND.' I SAID TO THE FO; 'THAT ISN'T US.' I NOTICED THE JET CLRING AT TXWY Y4 WHILE AT 50 FT AGL IN THE LNDG FLARE; SO I LANDED; EXITING THE RWY AT TXWY Y2. ON ROLLOUT WE HEARD ACR X (THE ONE WHO HAD BEEN GIVEN THE GAR) STATE THAT HE WAS; IN FACT; HOLDING SHORT FOR TKOF. I BELIEVE THAT THE CTLR; UPON SEEING THE CONFLICT; ISSUED THE MISS TO US BUT USED THE WRONG CALL SIGN. I DO NOT KNOW IF A LOSS OF SEPARATION OCCURRED. A CONTRIBUTING CONFUSION FACTOR MAY HAVE BEEN THAT WE WERE USING A RADIO CALL SIGN OF ACR Z INSTEAD OF THE USUAL ACR Y.