AN ACR LTT ON TKOF ROLL SAW ANOTHER ACR LTT ON A GAR PASS OVERHEAD IN CLOSE PROX. DEPARTING LTT MAINTAINED VISUAL SEPARATION FROM THE OTHER ACFT UNTIL GIVEN A TURN BEHIND IT.

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FOLLOWING A B757 CLRED INTO POS AND HOLD ON IAD'S RWY 19R. WITH THE CLRNC WAS INFORMED A 'BEECH' ON A 9 MI FINAL FOR OUR RWY. FIGURED THAT WOULD BE OK. IT WASN'T. AFTER APPROX 1 MIN 45 SECONDS IN POS; CLRED FOR TKOF. JUST PRIOR TO ROTATION; NOTICED A B-1900 WAS OVERFLYING US AT 200-300 FT AGL. THE BEECH BEGAN A R TURN SO WE CONTINUED THE TKOF. UNFORTUNATELY; THE BEECH STOPPED THE TURN AND FLEW PARALLEL TO THE RWY; APPROX 100 FT TO THE W OF RWY CTRLINE. ALTHOUGH WE HAD BEEN CLRED FOR TKOF WITH A R TURN TO 240 DEG HDG; WE MAINTAINED RWY HDG; AS SUCH A R TURN WOULD HAVE POINTED US TOWARDS THE BEECH 1900. INFORMED THE TWR WE WOULD MAINTAIN RWY HDG DUE TO THE ACFT TO OUR R. CTLR ACKNOWLEDGED AND INSTRUCTED US TO TURN TO SW WHEN CLR OF THE BEECH; WHICH WOULD MAINTAIN 1500 FT MSL. IN A LATER MEETING WITH TWR PERSONNEL; INFORMED THAT THE TWR HAD ISSUED THE GAR INSTRUCTION TO THE BE1900 SHORTLY AFTER OUR TKOF CLRNC. ALTHOUGH THE BEECH AND OUR ACFT MAINTAINED SAFE SEPARATION VISUALLY; FEEL: SIT SHOULD NOT HAVE BEEN ALLOWED TO DEVELOP AS FAR AS IT DID; AN EARLIER ATC CALL FOR GAR; OR 360 DEG TURN ON FINAL WOULD HAVE BEEN IN ORDER. ONCE THE PROB DEVELOPED THE TWR SHOULD HAVE BEEN ACTIVE IN PROVIDING BETTER SEPARATION; AS IT WAS; THE PLTS WORKED IT OUT FOR OURSELVES. IF I HAD IT TO DO OVER AGAIN; WOULD HAVE ABORTED THE TKOF. AS IT WAS; FEEL NO REGS BENT OR BROKEN; BUT SIT NOT WHAT IT SHOULD BE BY ANY MEANS.