AN E120 CREW; DEPARTING SFO; WAS BELIEVED; BY ATC; TO BE ANOTHER COMPANY ACFT WITH SIMILAR CALL SIGN AND GIVEN CLRNC TO TAKE OFF; EXPECTING A L TURN AFTER DEP. WHEN A R TURN WAS INITIATED; A QUERY WAS FORTHCOMING FROM ATC.

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WHEN WE INITIALLY CALLED SFO GND FOR TAXI; THEY ASKED US IF WE HAD OBTAINED A CLRNC; AND WE INFORMED THEM THAT WE HAD AND INFORMED THEM OF OUR ASSIGNED SQUAWK. THEY THEN INSTRUCTED US TO TAXI TO RWY 1L. UPON HOLDING SHORT OF RWY 1L; THE CTL TWR SAID 'ACR X ABCD; TAXI INTO POS AND HOLD.' AS WE WERE THE ONLY ACFT AT THE RWY; WE REPLIED 'ACR X ABCD INTO POS AND HOLD' (ASSUMING THAT HE HAD MISTAKEN OUR FLT NUMBER). I NOTED THAT THERE WAS AN ACR Y B737 HOLDING POS ON RWY 1R AND MADE A MENTAL NOTE THAT HE HAD NOT GIVEN US THE 'MAINTAIN RWY HDG' INSTRUCTION AS THEY NORMALLY DO WHEN WE USE RWY 1L AS THE DEP RWY FOR THE SFO 8 DEP (AND THERE IS TFC DEPARTING RWY 1R). THE CTLR NEXT STATED 'ACR X ABCD CLRED FOR TKOF RWY 1L;' AND WE AGAIN REPLIED 'ACR X ABCD CLRED FOR TKOF;' BUT WE DID NOT FORCE A CLARIFICATION OF THE FLT NUMBER AS WE SHOULD HAVE. AT THIS POINT; ACR Y WAS STILL HOLDING POS ON RWY 1R; BUT AT SOME POINT DURING OUR TKOF ROLL THEY WERE APPARENTLY CLRED FOR TKOF (NEITHER THE FO NOR MYSELF ACTUALLY HEARD THIS TKOF CLRNC; I GUESS BECAUSE WE WERE FOCUSING ON OUR TKOF AND CALLOUTS). AT 400 FT AGL; I ASKED 'CLR R?' AND THE FO LOOKED AND REPLIED 'CLR R' (HE COULD NOT SEE THE ACR Y AS THE B737 WAS TOO FAR BEHIND OUR ACFT). AS I BEGAN TO MAKE MY R TURN TO 030 DEGS; TWR ASKED IF WE WERE ON THE REBAS 3 AND WE REPLIED THAT WE WERE ASSIGNED THE SFO 8. HE INSTRUCTED US TO MAINTAIN RWY HDG; AND WE COMPLIED. ACR Y ASKED 'WHAT WAS THAT ALL ABOUT?' AND THE CTLR REPLIED THAT THERE HAD BEEN A MIX-UP OF CALL SIGNS. THERE WAS NEVER ANY TCASII ALERT OR EVASIVE ACTION TAKEN AND WE NEVER HAD VISUAL ON THE B737. OBVIOUSLY WE SHOULD HAVE CLARIFIED OUR FLT NUMBER PRIOR TO COMMENCING TKOF.