RPTED ACFT GAR FOR RWY 25R AND FLEW OVER RPTR'S ACFT HOLDING IN POS ON THE SAME RWY.

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WE WERE CLRED IN POS AND HOLD ON RWY 25R BY THE LCL CTLR. WHILE IN POS AND HOLD ACFT DOWN-FIELD WAS CLRED TO CROSS OUR RWY AND 'CONTACT GND ON OTHER SIDE.' WE HELD IN POS FOR APPROX 2 MINS. THE NEXT THING WE HEARD WAS 'GAR.' WE HEARD A VOICE SAY 'WE DIDN'T SEE HIM EITHER.' WE SAW A BOEING JET EXECUTING A MISSED APCH OVER THE TOP OF US. THE TWR THEN SWITCHED IT TO APCH FREQ AND APPROX 20 SECONDS LATER CLRED US FOR TKOF. TKOF AND DEP WERE NORMAL WITH NO FURTHER PROBS. WE HAD NOT HEARD ANY XMISSIONS TO THE OTHER JET OR HAD HEARD HIM BEING CLRED TO LAND. AFTER DEP I ASKED THE TWR WHETHER WE NEEDED TO TALK TO SOMEONE ABOUT WHAT HAPPENED AND HE STATED 'NO; BUT I NEED TO TALK TO SOMEONE.' SUPPLEMENTAL INFO FROM ACN 385845: TWR CTLR SEEMED DISTRACTED WHEN WE FIRST CONTACTED HIM AFTER SWITCHING FROM GND WHEN NEARING THE HOLD SHORT FOR RWY 25R. AT THAT HR; THE GND CTLR SEEMED TO BE WORKING MORE THAN ONE FREQ. WE KNOW THAT DURING SLACK PERIODS CTLRS ARE GIVEN TWO OR MORE FREQS/JOBS. I BELIEVE A CONTRIBUTING FACTOR IN THIS INCIDENT COULD HAVE BEEN A TWR CTLR GIVEN TOO MANY 'UNRELATED' DUTIES; WHERE HE COULDN'T GIVE HIS FULL ATTN TO CTLING HIS AIRSPACE/RWYS.