TWR HAD AN ACR MLG TAXI INTO POS AND HOLD ON RWY 25R; WHILE ON THE RWY THEY HEARD AND SAW A DH6 PASS OVERHEAD IN CLOSE PROX. THE MLG WAS NOT GIVEN A TA; NOR WAS THE DH6 TOLD ABOUT THE MLG ON THE RWY. RPTR FOUND OUT AFTER TKOF THAT THE DH6 WAS A PRACTICE APCH.

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WE HAD TAXIED TO RWY 25R AT LAS. WE SWITCHED TO TWR AND WERE CLRED INTO POS ON RWY 25R. AFTER WE TOOK THE RWY; THE TWR INFORMED US THAT WE WOULD BE HOLDING FIX FOR SEVERAL MINS DUE TO DEPS ON RWY 19. I WAS NOT CONCERNED SINCE I KNEW THAT ARRS WERE GOING TO RWY 25L AND THE RWYS 19'S. THE TWR FREQ WAS VERY BUSY AND I THOUGHT I HEARD SOMEONE CHK IN WITH THE TWR ON A PRACTICE APCH. I MISSED THE RWY THAT THEY WERE FLYING THE APCH TO AND I ASKED MY FO IF HE HEARD WHICH RWY THEY WERE FLYING THE PRACTICE APCH TO. HE ALSO HAD NOT HEARD WHICH RWY HE WAS GOING TO. I TRIED TO BREAK IN ON THE FREQ TO ASK WHERE THIS ACFT WAS BUT WAS UNSUCCESSFUL. ABOUT THIS TIME AN ACFT SHOWED UP ON TCASII DIRECTLY BEHIND US; 300 FT ABOVE AND DSNDING. I WAS ABOUT READY TO TAXI CLR OF THE RWY WHEN I HEARD AND COULD FEEL THE VIBRATION OF THE TWIN OTTER AS HE WENT MISSED APCH OVER THE TOP AND JUST TO THE L OF MY ACFT. WHEN HE CLRED; TWR CLRED US FOR TKOF AND DIDN'T SAY A WORD ABOUT THE TWIN OTTER. WE GOT THE INFO ON HIM AFTER TKOF.