C172 AND LOW WING ACFT HAD NMAC IN MYF CLASS D AIRSPACE.

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ON INITIAL CALL-UP TO TWR CTL AT MYF; I WAS TOLD TO SQUAWK XXXX ON MY XPONDER AND MAKE A STRAIGHT-IN APCH TO RWY 28L. I PROCEEDED AS REQUESTED. AT APPROX 5 NM E OF THE RWY; I HEARD A CLRNC OR SOME OTHER INSTRUCTION GIVEN TO ANOTHER PLANE FOR RWY 28L. AS A PRECAUTION; AND NOT HAVING HEARD FURTHER INSTRUCTION FOR ME FROM THE TWR; I CONTACTED THE TWR TO CHK ON MY CLRNC; AND CONFIRMED THAT I WAS CLRED FOR RWY 28L. THE CTLR MENTIONED TO THAT HE HAD TROUBLE CONTACTING ME ON MY CURRENT RADIO. I THEN SWITCHED TO MY #2 RADIO; AND CONFIRMED THAT THE TWR COULD READ ME. I COULD READ THE TWR ON BOTH RADIOS. APPROX 1/4 NM OR LESS FROM THE RWY ON FINAL; I HEARD THE TWR SAY THAT 2 ACFT WERE ATTEMPTING TO LAND ON THE SAME RWY AT THE SAME TIME. THE TWR THEN ISSUED A TFC ALERT. I LOOKED TO MY L IN TIME TO SEE A LOW WING AIRPLANE TURN AWAY FROM ME AND HEAD S. HE APPEARED TO HAVE BEEN FLYING A NONSTANDARD APCH. HE SHOULD HAVE BEEN BEHIND ME ON FINAL; OR SHOULD HAVE EXTENDED HIS DOWNWIND UNTIL IT WAS SAFE TO TURN BASE. HE SHOULD NOT HAVE BEEN ON MY L; FLYING A BASE APCH TO THE RWY. AS HIS PLANE VEERED AWAY; I HEARD HIM SAY 'I (MEANING HIM) HAD CLRNC.' I PROCEEDED TO LAND SAFELY. ON THE GND I CHKED BOTH RADIOS WITH GND CTL. THE RADIOS WORKED FINE. I BELIEVE THAT TWR CTL LOST AWARENESS OF THE 2 PLANES IN THE PATTERN FOR RWY 28L. I THINK THAT IF THE TWR HAD BEEN MORE ATTENTIVE; A SIMPLE INSTRUCTION TO EITHER PLANE COULD HAVE PREVENTED THIS CLOSE CALL. ALSO; THE LOW WING ACFT SHOULD NOT HAVE ATTEMPTED TO LAND; EVEN WITH CLRNC ON RWY 28L; IF THERE WAS ANOTHER PLANE BELOW IT AND ON FINAL APCH. HE SHOULD HAVE BEEN MORE VIGILANT. A CONTRIBUTING FACTOR MAY HAVE BEEN RADIO PROBS; BUT THERE WAS ADEQUATE TIME -- ONCE RADIO COMS WERE ADDRESSED -- TO AVOID THE CONFLICT THAT OCCURRED.