SMA ON STRAIGHT IN APCH HAS NMAC WITH AN SMA.

Narrative

I CONTACTED SDL TWR APPROX 10 MI OUT FOR A STRAIGHT IN APCH RWY 21. I WAS TOLD TO CONTINUE ON. AT THIS TIME I SAW AN SMA Y LOW AND AHEAD OF ME; MAYBE 3-4 MI. SDL TWR SPOKE TO AN ACFT; PERHAPS THIS ONE; AND ASKED IF ACFT WAS E OR W OF FINAL APCH FOR RWY 21. PLT SAID; 'AH; I DON'T KNOW; MAYBE W.' AS I GOT CLOSER; I WAS TOLD TO CONTINUE INBOUND. APPROX 3 MI OUT; I WAS ASKED TO SLOW DOWN DUE TO AN SMA TWIN THE PATTERN; AND I WAS TOLD THE SMA TWIN WAS MY TFC. I DID NOT SEE THE TFC AND TOLD THE TWR THIS. I WAS TOLD TO CONTINUE INBOUND. AT 1 1/2 - 2 MI OUT; AN SMA Y FLEW UNDER AND IN FRONT OF ME; CUTTING OFF MY APCH. HE WAS NOT TALKING TO TWR AT THAT TIME. HE APCHED FROM MY R. MY COPLT SPOTTED HIM FIRST. IF NOT FOR MY COPLT; I'M NOT SURE WHEN I WOULD HAVE SEEN HIM. AT THIS POINT; I IMMEDIATELY INITIATED A L 360 DEG TURN AND INFORMED TWR SIMULTANEOUSLY. AFTER I WAS ON THE GND; I CALLED THE TWR ON MY RADIO AND ASKED WHERE THAT AIRPLANE CAME FROM? THE CTLR TOLD ME THAT HE HAD TOLD ME TO FOLLOW THAT PLANE IN. ALL I REMEMBER HEARING WAS TO SLOW DOWN FOR TFC IN THE PATTERN; WHICH WAS AN SMA TWIN; NOT AN SMA Y. MY COPLT DOES NOT REMEMBER BEING ADVISED OF THE OTHER ACFT EITHER. I GUESS THAT I WAS SURPRISED THAT THE CTLR DID NOT ASK ME TO MAKE A L 360 DEG TURN BEFORE I DID. IN OTHER WORDS; SEE THE SIT DEVELOPING BEFORE I COULD SEE IT; ALTHOUGH I REALIZE THE ULTIMATE RESPONSIBILITY IS MINE FOR MY OWN SAFETY.