B727 CREW FOLLOWED WRONG ACFT ON THE VISUAL AT TJSJ.

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ON MAR/XA/00 AT ABOUT XA50Z WE WERE INBOUND FROM ST KITTS TO SJU; PR. ZSU HANDED US OFF TO SJU APCH CTL (120.9). ONCE WE GOT IN THE TFC AREA THEY HANDED US OFF TO 119.4. MIDFIELD DOWN RWY 10 CONTACT WAS MADE FOR A VISUAL INTO RWY 10 WHICH WAS DENIED DUE TO TFC. OUR HDG WAS 280 DEGS. THE CTLR THEN GAVE US A HDG OF 330 DEGS DOWN TO 2000 FT. 12 MI OUT OF SJU; ADVISED US OF TFC (B757). THE FO AND FE SAW TFC 1-2 O'CLOCK POS; WHICH APPEARED TO THEM TO BE A B757. INITIAL CONTACT WAS MADE THEN LOST DUE TO CLOUDS THEN SPOTTED AGAIN. AT THAT POINT; WE CALLED OUT TFC AND WERE CLRED FOR THE LAGOON VISUAL RWY 8 APCH BEHIND THE B757. WE THEN TURNED TO FOLLOW WHAT APPEARED TO BE THE TFC WE WERE SUPPOSED TO FOLLOW IN. APCH CTL THEN RADIOED THAT WE WERE FOLLOWING THE WRONG ACFT. AT THAT TIME WE SPOTTED THE B757 WE WERE ORIGINALLY SUPPOSED TO FOLLOW. WE THEN MADE IMMEDIATE MANEUVERS TO AVOID ANY KIND OF INCIDENT WITH ANY ACFT WITH THE HELP OF THE CTLR. WE NEVER CAME ANY CLOSER THAN 6 OR 7 MI OF ANY ACFT AND BELIEVE THAT HAVING TCASII WOULD HAVE AVOIDED THIS SIT ENTIRELY.