CPR C560 FLC CLBS TO AN ALT MEANT FOR ANOTHER CPR ACFT WITH A SIMILAR CALL SIGN. CTR CTLR MISSES THE ERROR AT FIRST AND AFTER C560 'X' LEVELS AT 16;000 FT; THE CTLR CORRECTS THE SIT. CTR WAS VERY BUSY.

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ON A ROUTINE FLT FROM MMU TO ACY WE WERE WORKING WITH A VERY BUSY ZNY. ON THE SAME FREQ ANOTHER COMPANY ACFT (SAME CALL SIGN; DIFFERENT FLT NUMBER) DEPARTED FROM ANOTHER NY ARPT. BOTH ACFT WERE IN A SIMILAR PHASE OF FLT (DEP; ENRTE CLB). WE HAD CLBED TO 8000 FT AND RECEIVED INSTRUCTIONS TO CLB TO 16;000 FT. I WAS THE PF AND THE PNF READ BACK THE CLB CLRNC. NO AMENDMENTS FOLLOWED THIS READBACK. AFTER LEVELING AT 16;000 FT AND FLYING IN CRUISE FOR SOME 3-5 MINS THE CTLR NOTIFIED US THAT WE HAD TAKEN INSTRUCTIONS FOR THE OTHER ACFT AND THAT WE SHOULD DSND BACK TO 8000 FT. I DID AS INSTRUCTED AND CONTINUED TO ACY. THIS SIT POINTS TO THE EVER PRESENT PROB OF SIMILAR CALL SIGNS AND CONGESTED FREQS. THE CTLR WAS VERY BUSY WITH LITTLE OR NO TIME BTWN XMISSIONS. THE PNF READ BACK THE INSTRUCTIONS; PAUSED BRIEFLY; THE CTLR CONTINUED WITH OTHER ACFT AND WE EXECUTED OUR CLB INSTRUCTIONS. THE CHAIN OF EVENTS FAILED IN TWO PLACES: 1) NEITHER MYSELF NOR THE PNF CAUGHT THE FACT THAT A DIFFERENT TAIL NUMBER THAN OURS MIGHT HAVE BEEN ISSUED. WE FELT THAT THE CALL WAS ISSUED TO US AND NEVER EVEN QUESTIONED THE FACT THAT IT WAS NOT. 2) THE CTLR DID NOT QUESTION THE READBACK. POSSIBLY THE CTLR CONFUSED ONE ACFT FOR ANOTHER; OR READ THE CLRNC FOR A DIFFERENT NUMBER; OR WE DID NOT HEAR THE CTLR RIGHT; HOWEVER THE FACT REMAINS THAT AFTER VERIFYING THE CLRNC; NO AMENDMENTS WERE MADE. LUCKILY; NO TFC CONFLICTS RESULTED.