A B757 FLC HEARD THEIR RADIO CALL SIGN CLRED INTO POS AND HOLD DURING THE TKOF ROLL AT LGA.

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ON AUG/XA/99; I WAS THE FO AND PF ON FLT FROM LGA TO MIA. TWR CLRED US INTO POS AND HOLD ON RWY 13; WHICH I ACKNOWLEDGED. AS THE CAPT WAS TAXIING ONTO THE RWY; THE TWR CLRED US FOR TKOF ON RWY 13; WHICH I AGAIN ACKNOWLEDGED. WHEN THE CAPT HAD THE ACFT LINED UP; HE PASSED CTL OF THE PLANE TO ME. I TOOK CTL ON THE ROLL AND BEGAN PUSHING THE PWR UP TO TKOF PWR. JUST AS I LET GO OF THE THROTTLES IN THE 'FLEX' TKOF PWR POS; TWR SAID 'ACFT X; CLRED INTO POS AND HOLD RWY 13.' THE CAPT IMMEDIATELY BEGAN A PWR REDUCTION WHILE SIMULTANEOUSLY TELLING TWR WE HAD ALREADY RECEIVED TKOF CLRNC. THE TWR IMMEDIATELY RESPONDED WITH 'ROGER; YOU ARE STILL CLRED FOR TKOF.' THE CAPT IMMEDIATELY PUT THE THROTTLES BACK INTO THE TKOF PWR POS. I ESTIMATE THE PWR FLUCTUATION HAPPENED AT ABOUT 50 KTS AND BECAUSE IT HAPPENED SO QUICKLY; FELT OUR TKOF ROLL WAS NOT ADVERSELY AFFECTED. I RETAINED DIRECTIONAL CTL THROUGHOUT THE PWR CHANGE AND INITIATED A NORMAL TKOF AND CLBOUT. DURING THE CLBOUT; THE TWR CTLR APOLOGIZED FOR THE ERROR AND SAID SOMEONE HAD MOVED THE WRONG SLIP. MY PERCEPTION WAS THAT WE AVOIDED A POTENTIALLY DANGEROUS SIT OF STOPPING ON THE RWY WHEN ANOTHER ACFT HAD BEEN CLRED TO LAND ON THE XING RWY OF RWY 22. WE DID NOT USE ANY DECELERATION TECHNIQUES SUCH AS SPOILERS OR BRAKES BUT MERELY DID NOT ACCELERATE AS FAST AS POSSIBLE; THEREFORE CONTINUING THE TKOF WAS THE SAFEST COURSE OF ACTION.