ACR CROSSES HOLD SHORT LINE AND CAUSES A GAR.

Narrative

WHILE TAXIING FOR TKOF AT THE RDU ARPT; WE WERE PERFORMING OUR USUAL TAXI AND TKOF CHKS. IT WAS NIGHTTIME; AROUND XA00; WITH RWYS 5L&R IN USE UNDER SINGLE TWR CTL; SINGLE GND CTL. MY CAPT WAS OFF FREQ; TALKING TO THE COMPANY; WHILE I WAS MONITORING GND CTL TO SWITCH OVER TO TWR. I DID SO; SWITCHING OVER TO THE TWR FREQ; MY CAPT STILL OFF FREQ WITH THE COMPANY; AND HEARD WHAT I THOUGHT TO BE A POS AND HOLD COMMAND FROM THE TWR CTLR FOR RWY 5R. I READ BACK THE CLRNC FOR A POS AND HOLD ON RWY 5R;; HEARD NO FEEDBACK FROM THE TWR; WHICH IS TYPICAL; AND TOLD THE CAPT WE WERE POS AND HOLD; JUST AS HE CAME BACK ON FREQ. WE PROCEEDED TO DO SO; AND JUST AS WE GOT TO THE HOLD LINE FOR THE RWY; WE WERE TOLD TO HOLD SHORT BY THE TWR; CAUSING US TO STRADDLE THE HOLD SHORT LINE. TWR ASKED IF WE HAD IN FACT CROSSED THE LINE; WHICH WE ANNOUNCED WE HAD. HE THEN PROCEEDED TO TELL THE ACFT ON A 2 MI FINAL TO GO AROUND. HE THEN CLRED US INTO POS ON RWY 5R; AND EVENTUALLY FOR TKOF. CONTRIBUTING FACTORS FOR THIS OCCURRENCE WERE: 1) SINGLE TWR FOR SIMILAR SOUNDING PARALLEL RWYS WITH ACFT WITH SIMILAR SOUNDING CALL SIGNS; AT NIGHT WITH STATIC ON THE FREQ. WE ALSO DIDN'T GET A READBACK CONFIRMATION ON THE POS AND HOLD COMMAND WE MISTAKENLY HEARD FROM THE TWR. HE COULD HAVE EASILY TOLD US TO HOLD SHORT WELL BEFORE THE INCIDENT OCCURRED WHEN WE READ BACK THE MISHEARD CLRNC IF HE WASN'T PREOCCUPIED WITH 2 RWYS.