ACFT CLRED FOR TKOF; WRONG ACFT DOWNFIELD STARTS TKOF ROLL. PLTDEV.

Narrative

WHILE TALKING TO ONT TWR PRIOR TO TKOF THERE WAS CONFUSION BTWN OUR ACFT AND AN SMT Y WITH A VERY SIMILAR CALL SIGN. WE WERE CLRED TO CROSS 8L AND HOLD SHORT OF 8R AT TAXIWAY 20. WHILE HOLDING SHORT THE TWR SAY 'ACR X; CLRED FOR TKOF.' I RESPONDED 'ACR X CLRED FOR TKOF ON 8R.' THERE WAS NO CORRECTION BY THE TWR UNTIL WE WERE ALMOST LINED UP WITH RWY HDG; AT WHICH TIME I HEARD THE TWR SAY 'ACR X CANCEL TKOF CLRNC.' SINCE WE WERE ALREADY ON THE RWY; I SAID 'ROGER ACR X IS POS AND HOLD; CANCELLING CLRNC.' AT THAT TIME I NOTICED THE SMT Y HALF WAY DOWN THE RWY FACING TOWARD US. THE TWR THEN SAID 'YOU ARE NOT CLRED POS AND HOLD.' AT THIS POINT IT BECAME CONFUSING WHETHER TWR WAS TALKING TO US OR THE SMT Y AS I HEARD THE OTHER ACFT RESPONDING TO CALLS I BELIEVED WERE MEANT FOR US. AS WE STARTED TO MANEUVER OFF THE RWY; THE SMT Y TAXIED CLR OF THE RWY; AND TWR CLRED US FOR TKOF ON 8R. I COULD NOT DIFFERENTIATE ANY DIFFERENCE IN 'CALL SIGNS' FROM TWR XMISSIONS BTWN OUR ACFT AND THAT OF THE OTHER ACFT. APPARENTLY THE TWR HAD TROUBLE TELLING THE DIFFERENCE AS WELL SINCE IMMEDIATE CORRECTIONS DID NOT FOLLOW OUR XMISSIONS AND RESPONSE TO TWR CALLS. I WAS NOT AWARE OF THE EXISTENCE OF THE OTHER ACFT UNTIL WE TOOK THE RWY IN PREPARATION FOR TKOF.