MULTIPLE RWY OP INTERSECTING RWYS. ACR X MADE UNAUTHORIZED TKOF WHEN ACR Y SIMILAR NUMBERS WAS CLRED FOR TKOF. PLTDEV.

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WE WERE ACR FLT X AND WERE CLRED TO TAXI TO RWY 35 INTXN A. UPON REACHING 35 AT A; WE WERE #1 AT THAT RWY AND SWITCHED TO TWR FREQ. AFTER A FEW ARRS; TWR INSTRUCTED ACR Y INTO POS AND HOLD AT RWY 35 INTXN A. I REPLIED ACR X ON AND HOLD 35A; THINKING THE CTLR MIXED THE FLT NUMBER UP WHICH IS A COMMON OCCURRENCE IN PHL. TWR THEN INSTRUCTED ACR Y CLR FOR TKOF 35 FLY RWY HDG. MY REPLY WAS HDG CLR FOR TKOF 35; ACR X; STRESSING THE X. NO ACKNOWLEDGEMENT OF THE DIFFERENT FLT NUMBERS WAS MADE FROM THE CTLR. AFTER TKOF THE TWR CTLR INSTRUCTED ACR Y TO CONTACT DEP. FOR THE 3RD TIME I STRESSED THE CORRECT FLT NUMBER. AT THIS TIME THE CTLR REALIZED THE DIFFERENCE AND SAID 'THAT WAS ACR X THAT JUST DEPARTED.' I REPLIED AFFIRMATIVE; AND THE CTLR RETURNED WITH ACR X; CONTACT DEP; USING THE CORRECT CALL SIGN. AFTER LOOKING INTO IT; MY CAPT AND MYSELF FOUND OUT THAT THERE WAS AN ACR Y TAXIING AT THE SAME TIME. HOWEVER; THEY WERE GOING TO RWY 27L BECAUSE THEY WERE UNABLE TO ACCEPT RWY 35 DUE TO RESTRICTIONS. IT WAS OBVIOUS TO US THAT THERE WAS A LACK OF COM BTWN THE GND CTLR AND THE TWR CTLR; WHICH RESULTED IN THE TWR CTLR NOT KNOWING WHICH ACFT WAS AT RWY 35A. KNOWING WE WERE THE ONLY ONES AT 35A; AND THE CTLR CALLING US Y INSTEAD OF X; WE ASSUMED HE WAS TALKING TO US AND WAS JUST MIXING UP THE FLT NUMBERS. AS MENTIONED BEFORE; THIS IS A COMMON OCCURRENCE AT PHL. I ALSO BLAME MY COMPANY FOR HAVING 2 FLTS WITH SIMILAR FLT NUMBERS OPERATING AT THE SAME TIME. POSSIBLE SOLUTIONS ARE FOR THE CTLRS TO LISTEN MORE CAREFULLY WHEN FLTS READ BACK INSTRUCTIONS. THIS WAS DONE TWICE WITH THE CORRECT NUMBER BEFORE THE CTLR REALIZED IT. IN ADDITION; AIRLINES SHOULDN'T ASSIGN FLTS WITH SIMILAR FLT NUMBERS. THIS IS CONFUSING TO THE PLTS AS WELL AS THE CTLRS.