TCASII ALERT BTWN ACR X AND UGA Y.

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WE WERE CLRED FOR TKOF ON RWY 9L AT THE MIA ARPT. THE INITIAL CLRNC WAS TO TURN R TO A HDG OF 120 DEGS AND MAINTAIN 5000 FT. ON THAT HDG THE ACFT WAS CLEANED AND IN A CLB. ATC ASSIGNED HDG 090 DEGS FOR TFC. NOT NOTICING ANY TARGETS PRIOR TO THE ATC CLRNC; I LOOKED AGAIN AND FOUND A TARGET AT ABOUT THE 11 O'CLOCK POS AND 200 FT BELOW US. THE TARGET WAS IN THE INNER CIRCLE AT THIS TIME. THE TCASII EQUIP HAD ONLY 1 SCALE. AS I TURNED TO A HDG OF 090 DEGS; I LOOKED OUT TO MY R EXPECTING TO SEE THE TFC. I DID; AND AT ABOUT 1-2 O'CLOCK WHERE IT WOULD HAVE TO BE. IN THE MEANTIME THE TCASII GAVE AN RA AND COMMANDED A DSCNT OF ABOUT 2000 FPM; HOWEVER; THE TARGET SEEMED TO REMAIN IN THE SAME POS EVEN AFTER THE TURN. IN EVALUATING WHAT I WAS GIVEN I DECIDED NOT TO ACT IN ACCORDANCE WITH THE TCASII FOR THE FOLLOWING REASONS: 1) I HAD A VISUAL ON WHAT I BELIEVED WAS THE INITIAL TARGET. 2) THE SEEMINGLY SUDDEN APPEARANCE OF THE TARGET IN THE INNER CIRCLE. 3) THE TARGET DID NOT MOVE IN ACCORDANCE WITH OUR 30 DEG HDG CHANGE. 4) I WOULD HAVE HAD TO MANEUVER THE ACFT CLOSE TO THE TERRAIN AND MAKE A RADICAL ATTITUDE CHANGE FROM A 2500 FPM RATE OF CLB TO A 2000 FPM RATE OF DSCNT. 5) WE WERE NOT ISSUED ANY OTHER TA BY ATC. 6) THE CAPT WAS HESITANT IN TAKING ANY ACTION AS WELL. 7) THE 'SEE AND AVOID' PRINCIPLE PREVAILED AT THE TIME AND NO OTHER TFC WAS IN SIGHT. IT IS MY BELIEF THAT THE TCASII LOCKED ON THE INITIAL TARGET AND DID NOT IDENT OUR TURN SINCE IT CAN ONLY PROVIDE US WITH VERTICAL SEPARATION.