MLG ACFT CLRED FOR TKOF AND ON TKOF ROLL SAW ANOTHER ACFT WHICH HAD ERRONEOUSLY ENTERED THE RWY. RPTR FLC WERE ABLE TO ROTATE PRIOR TO XING THE OTHER ACFT.

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THERE WAS A LIGHT RAIN FALLING AND THE RWY WAS SLICK. AFTER A LONG TAXI WE FINALLY FOUND THE END OF RWY 28R AND RWY 28L AT SFO. ATC HAD US CROSS RWY 28L; HOLD SHORT RWY 28R. AS WE APCHED OUR WHEELS-UP TIME FOR LAX; WE WERE GIVEN POS AND HOLD RWY 28R. BRAKES WERE SET AWAITING TKOF CLRNC. THERE WAS ANOTHER ACR X FLT ON THE TWR FREQ AT THE SAME TIME. WE WERE CLRED FOR TKOF. AS THE FO ADDED TKOF THRUST (IT WAS HIS LEG TO LAX) TWR CLRED ACR X TO CROSS RWY 28L AT TXWY P; HOLD SHORT RWY 28R. AFTER SETTING TKOF THRUST; AND CALLING 80 KTS THRUST SET; ACR X CAUGHT MY EYE ON THE TKOF ROLL. THEY HELD SHORT OF RWY 28R AS INSTRUCTED. AS I FOCUSED DOWN THE RWY OF RWY 28R SOMEWHERE AROUND 100-110 KTS. I SAW A TWIN ENG BARON BEECHCRAFT ON THE RWY IN FRONT OF US! AT THAT MOMENT I SAID; 'ACFT ON THE RWY -- ROTATE!' SECONDS AFTER THAT THE TWR SAID; 'OH SHIT!' HEARING THAT I TOLD THE TWR; 'WE HAVE HIM IN SIGHT; WE'RE ROTATING.' THE BARON WAS APPROX 800-900 FT IN FRONT OF US PRIOR TO MY CALL; 'ROTATE.' WE WERE AIRBORNE BY THE INTXN OF RWY 28R AND RWY 1L; AS THE BARON WAS TRYING TO CLR THE ACTIVE RWY AT TXWY E AND TXWY C. THE FO FLEW THE ACFT NICELY; WITH NO PROB AT ALL. MY ATTN WAS FOCUSED ON THE BARON. WE CLRED THE BARON BY 40- 50 FT IN THE AIR. THE FLT CONTINUED TO LAX NORMALLY WITHOUT FURTHER INCIDENT. COMMENTS: I'M GLAD CHANNEL 9 WAS OFF. THE BARON HAD THEIR LNDG LIGHTS ON. THIS HELPED ME SEE THE ACFT. WE WERE LIGHT AND ABLE TO TAKE OFF EARLY. HAD AN EXCELLENT FO; HE DID NOT LOSE HIS COOL. THE TWR DID NOT TELL US TO ABORT. IF THEY HAD; WHO KNOWS WHAT WOULD HAVE HAPPENED.