FLC RESPONDED TO CLRNC WITH READBACK. THEN TOLD HOLD SHORT OF RWY THOUGHT CLRED TO CROSS. SIMILAR CALL SIGNS.

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AFTER A NORMAL LNDG WE CLRED RWY 25L. TWR ISSUED TAXI INSTRUCTIONS AND WE ACKNOWLEDGED WHAT WE THOUGHT WE HEARD (FULL CALL SIGN AND INSTRUCTIONS). AS WE APCHED 25R; TWR TOLD US TO HOLD SHORT WHICH WE DID. WE QUERIED TWR AND TOLD HIM WE UNDERSTOOD WE WERE CLRED TO CROSS 25R. WE TOLD HIM THAT OUR READBACK ON THE FIRST SET OF INSTRUCTIONS WAS TO CROSS 25R. WHAT HAD OCCURRED IN ALL OF THESE EVENTS WAS THERE WAS ANOTHER ACFT WITH A SIMILAR SOUNDING CALL SIGN AND THE SAME FLT NUMBER ON THE FREQ AND HE WAS BEING ISSUED INSTRUCTION ALSO WHICH WE HEARD. TWR ISSUED US TAXI INSTRUCTIONS THEN GAVE ANOTHER SET OF INSTRUCTIONS TO ANOTHER ACFT. TWR MISSED OUR MISUNDERSTOOD READBACK ALTHOUGH TWR DIDN'T CATCH OUR MISTAKE ON THE READBACK HE DID CONTINUE TO MONITOR OUR TAXI AND REISSUED INSTRUCTIONS TO US. NO ONE IN THIS SYS CAN ASSUME ANYTHING. WE HAVE TO STAY ALERT NO MATTER HOW ROUTINE THE JOB BECOMES; AND WE HAVE TO USE FULL CALL SIGNS AND READBACK CLRNCS. HAD THE VISIBILITY BEEN DOWN IT COULD HAVE BEEN TOTALLY MISSED. TWR CTLR WAS DOING HIS PRIMARY JOB AND HAD TIME TO KEEP AN EYE ON THE SITUATION. THIS LEADS ME TO THE PROC I FIND AT SOME OTHER ARPTS I FLY INTO. EVEN THOUGH A CTLR'S WORKLOAD IS NOT HIGH AND SOMEONE MAKES THE DECISION TO COMBINE CLRNC; GND AND TWR TO 1 FREQ IT IS SETTING UP THE ENTIRE SYS FOR A POSSIBLE ACCIDENT. IF GND AND TWR WERE COMBINED IN MY SITUATION; TWR MAY NOT HAVE HAD TIME TO VISUALLY CHK ON ME AND MAKE SURE I WAS DOING WHAT I WAS INSTRUCTED TO DO.