AIR TAXI CLRED TO POS AND HOLD HAS SMA FLY OVERHEAD TO LAND.

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GND CTL CLRED ME TO TAXI TO RWY 1L VIA TAXIWAY DELTA; TO HOLD SHORT OF ECHO 5. ANOTHER COMPANY SMT WAS IMMEDIATELY BEHIND WITH THE SAME INSTRUCTIONS. I WAS CLRED BY TWR INTO POS TO HOLD; AS I MOVED ACROSS THE HOLD BARS; I COMPLETED TKOF CHKLIST AND TOOK A LOOK TO THE L (FINAL APCH). IT LOOKED CLR. AS I STOPPED MY ACFT ON THE CTRLINE; I SIMULTANEOUSLY HEARD THE TWR TELL ME TO HOLD SHORT AND SAW A SINGLE-ENG SMA PASS OVER MY ACFT. (I FOUND OUT LATER TALKING WITH THE TWR MGR THAT VISIBILITY FROM THE TWR CAB OF THAT AREA WAS SUCH THAT THE CTLRS COULD NOT SEE THAT I WAS ALREADY ON THE RWY AS INSTRUCTED). THE SMA TOUCHED DOWN ON THE RWY AN ESTIMATED 400-600 FT IN FRONT OF ME; THEN APPARENTLY LOST CTL OF HIS ACFT; VEERING INTO THE DITCH TO THE L AND THEN RECOVERING CTL FACING W; ON ECHO 2. (THE OPERATORS IN THE TWR DIDN'T SEE THIS EITHER). FOR A MOMENT I THOUGHT PERHAPS I HAD NOT BEEN CLRED ONTO THE RWY; AND ASKED THE CTLR IF HE WANTED ME TO CLR THE RWY. AS THE SMA TAXIED BY (S ON ECHO) APPARENTLY NONE THE WORSE FOR WEAR; THE TWR CLRED ME TO TKOF; AND THANKED ME FOR MY HELP. WHEN I ARRIVED BACK IN LAS VEGAS ON THE RETURN TRIP; GND CTL ASKED ME TO PHONE THE TWR. TALKING TO THE TWR MGR I: RECEIVED AN APOLOGY FROM THE MGR FOR THE INCIDENT; TWR OPERATOR BELIEVED SMA ALREADY DOWN AND CLR. LEARNED THAT VISIBILITY FROM THE TWR CAB OF THAT PARTICULAR AREA WAS VERY POOR. REALIZED THAT THE TWR OPERATOR HADN'T SEEN THE LNDG SMA LOSE CTL AND RECOVER. HEARD THAT MY PARTICULAR CTLR WAS EXPERIENCED BUT NEW IN THAT POS; AND THAT APPROPRIATE CHANGES WERE TO BE MADE IN PROCS AND PERSONNEL.