NMAC BETWEEN TWO SMALL ACFT IN TRAFFIC PATTERN AT FAA TWR ARPT. OPERATIONAL ERROR.

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THE PROB AROSE DURING A SERIES OF FULL-STOP LNDGS WITH TAXI-BACK TO THE 30R THRESHOLD EACH TIME. JUST PRIOR TO TKOF #5; THE LCL CTLR CHANGED FROM AN 'OLD HAND' (MALE) TO AN APPARENTLY LOW-TIME CTLR (FEMALE). TKOF; UPWIND; XWIND AND DOWNWIND WERE ROUTINE. RADIO COMS WERE LOUD AND CLR. WE WERE CLRED TO LAND FOR THE 5TH TIME AND BEGAN THE DSCNT; TURNED BASE; THEN BEGAN THE TURN TO FINAL. DURING THIS TURN; ON A HDG OF APPROX 270 DEGS; THE FEMALE CTLR CAME UP ON 120.7 WITH; 'OH; MY GOD!' SHE SAID NOTHING ELSE; AND I BEGAN A FULL POWER CLBING RIGHT TURN TO A HDG OF APPROX 330 DEGS. A FRIEND SEATED IN THE LEFT SEAT SAW SMA Y AT OUR ALT ABOUT 200' OFF TO OUR LEFT (APPROX TEN O'CLOCK); PLACED HIS HAND ON THE YOKE AND STEEPENED THE RIGHT TURN. CLBING THRU APPROX 700' MSL; THE FEMALE VOICE INSTRUCTED US TO PARALLEL THE RWY AND REENTER DOWNWIND. THE PATTERN AND LNDG WERE MADE W/O FURTHER INCIDENT. SITTING IN THE RIGHT SEAT; MAKING A CLBING RIGHT TURN; I NEVER DID SEE SMA Y WHICH WAS HIDDEN UNDER OUR LEFT WING. WE PROCEEDED TO MAKE ONE MORE PATTERN. WHILE IN FINAL; I HEARD A MALE VOICE ASK; 'DID WE CUT OUT THAT OTHER SMA?' THE FEMALE CTLR RESPONDED; 'NO; YOU WERE CLRED TO LAND.' AT NO TIME DID I HEAR ANY RADIO XMISSIONS TO ACFT Y; NOR DID MY FRIEND HEAR ANY RADIO XMISSIONS TO ACFT Y. (MY FRIEND IS A COMMERICAL PLT.) WE DON'T KNOW IF ACFT Y WAS MAKING A STRAIGHT IN APCH OR WHERE HE CAME FROM. WE WERE NOT INFORMED WHILE DOWNWIND THAT WE WERE '#2.' DURING OUR LAST PATTERN AND WHILE TAXIING IN ON GND FREQ; NO MENTION OF THIS INCIDENT WAS MADE BY ANY CTLR. NO PLTS WERE ASKED TO 'CALL THE TWR;' ALTHOUGH I HAVE FREQUENTLY HEARD THE TWR MAKE SUCH A REQUEST TO OTHER PLTS AFTER FAR LESS SERIOUS INCIDENTS. I DO NOT BELIEVE THAT 'OH; MY GOD!' IS IN THE LEXICON OF STANDARD ATC PHRASEOLOGY; AND THE FEMALE CTLR PROVIDED NO HELP WHATSOEVER IN ADVISING EITHER PLT(S) AS TO WHAT EVASIVE ACTION TO TAKE. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING: RPTR DID NOT REALIZE HE SHOULD TELL ATC IF HE WISHED TO FILE AN NMAC RPT. HE EXPECTED ATC TO CONTACT HIM LATER BUT HAS NOT HEARD FROM FAA REF THE INCIDENT. HAS HEARD SAME TWR CTLR ON LCL CTL POS SINCE INCIDENT AND SAYS CTLR MAKES A LOT OF MISTAKES IN CALL SIGNS; ETC. NOW BELIEVES THAT SMA Y WAS MAKING STRAIGHT IN APCH AND HAD BEEN CLRED TO LAND WHILE STILL SOME DISTANCE OUT FROM RWY; AND WHILE RPTR WAS ON GND CTL FREQ DURING PREVIOUS TAXI BACK OP. WOULD HAVE LIKED TO LISTEN TO TWR TAPES; BUT AVIATION ATTORNEY ADVISED HIM TO 'LET SLEEPING DOGS LIE;' AS THE FAA MIGHT TURN INCIDENT INTO CERTIFICATE ACTION AGAINST RPTR.