INSTRUCTOR IN C152 WITH SPI IN TFC PATTERN HAS NMAC WITH A MERLIN WHICH ENTERS TFC PATTERN INCORRECTLY AND IS HEAD ON WITH THE C152.

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INBOUND FROM SE OF ARPT; WE WERE INSTRUCTED TO ENTER ON 2 MI L 45 DEG TO RWY 21L. JUST AFTER RPTING 2 MI 45 DEG; A MERLIN RPTED 3 MI W OF PRC. MERLIN WAS INSTRUCTED TO ALSO ENTER ON 2 MI L 45 DEG. AS WE ENTERED DOWNWIND; PRC TWR INSTRUCTED US TO FOLLOW A DUCHESS AHEAD ON FINAL. WE MISIDENTED THE MERLIN (WHO WAS LOST) AS THE DUCHESS. ABOUT THEN; THE MERLIN RPTED ON 2 MI L 45 DEG. THEY WERE ACTUALLY ENTERING R DOWNWIND RWY 3R AND TURNED DIRECTLY INTO US. WE TURNED R; AT THE SAME TIME PRC TWR ASKED MERLIN TO SAY POS. THEY WERE ON R DOWNWIND ABEAM NUMBERS RWY 21L. TWR THEN 'POLITELY AS POSSIBLE' NOTIFIED THE MERLIN TO CIRCLE S UNTIL FURTHER ADVISED. WHY I SUBMITTED THIS ASRS FORM IS THAT WHEN ON GND FREQ THE GND CTLR ASKED THE MERLIN IF THEY KNEW WHAT THEY DID. THE MERLIN WAS TOTALLY UNAWARE THAT THEY WERE SE OF THE FIELD INSTEAD OF W AS RPTED; AND UNAWARE THAT THEY ENTERED THE PATTERN AND FLEW AGAINST THE TFC PATTERN FLOW. IF THIS HAD HAPPENED 1 MONTH AGO WHEN ALL 3 FLT SCHOOLS AT THIS AIRFIELD WERE IN FULL OP; THERE WOULD HAVE BEEN AN ACCIDENT. THERE ARE TIMES OF THE DAY THAT THE PATTERN IS SATURATED WITH TFC AND THE LAST THING THE PRC CTLRS NEED IS A FAST MOVING ACFT DOING THINGS THEIR WAY.