AN E120 CREW; DEPARTING SFO; WAS BELIEVED; BY ATC; TO BE ANOTHER ACFT WITH SIMILAR CALL SIGN AND GIVEN CLRNC TO TAKE OFF. THE MISTAKE WAS DISCOVERED WHEN SECOND ACFT WAS CLRED FOR TKOF USING PREVIOUS ACFT'S CALL SIGN.

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WE WERE CLRED TO TAXI TO RWY 1L IN SFO. WE WERE THE ONLY ACFT AT THE END OF RWY 1L WHEN WE GOT THERE. THERE WAS 1 OTHER SIMILAR CALL SIGN ACFT (ACFT Y) TAXIING OUT AT THE TIME; BUT THEY WERE STILL APPROX 2 MINS FROM REACHING THE END OF THE RWY. THE FO AND I BOTH BELIEVED WE HEARD OUR CALL SIGN WHEN TOLD TO TAXI INTO POS AND HOLD AND WE DID SO. AN ACR Z; B737; WAS CLRED FOR TKOF FROM RWY 1R AND WE WERE INSTRUCTED TO KEEP THEM IN SIGHT. WE THEN BELIEVE WE HEARD OUR CALL SIGN CLRED FOR TKOF FROM RWY 1L. ON THE CLBOUT; WE THEN HEARD TWR CLR OUR CALL SIGN INTO POS AND HOLD ON RWY 1L. (THE ONLY OTHER ACFT BEING THE SIMILAR SOUNDING CALL SIGN WHO HAD FINALLY REACHED THE END OF RWY 1L.) I INFORMED TWR WE WERE ON THE DEP. TWR ADVISED US TO MAINTAIN RWY HEADING AND ASKED IF WE STILL HAD THE B737 IN SIGHT (WE DID). WE WERE HANDED OFF TO DEP WITHOUT FURTHER INCIDENT. THE FO AND I STILL DON'T KNOW FOR SURE EXACTLY WHAT HAPPENED; BUT WE BELIEVE THE CLRNC FOR TKOF WAS FOR THE SIMILAR SOUNDING CALL SIGN TAXIING OUT; ALTHOUGH THEY WEREN'T EVER ON TWR FREQ YET. I THINK IT WAS A COMBINATION OF TWR FORGETTING WHICH ACFT WAS WAITING AT THE END OF RWY 1L AND US 'HEARING' WHAT WE EXPECTED TO HEAR. FOR BOTH INSTRUCTIONS (POS AND HOLD/CLRED FOR TKOF) WE RESPONDED WITH OUR CALL SIGN SO IF TWR HAD MEANT THEM FOR THE OTHER ACFT; HE DIDN'T CATCH THE READBACK EITHER.