EMBRAER 145 STOPPED TAXIING INTO POSITION AND HOLD AFTER FO NOTICED LNDG ACFT ON SHORT FINAL.

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WE WERE CLRED BY DFW TWR TO TAXI INTO POS AND HOLD ON RWY 35L SHORTLY AFTER AN ACR Z B717 DEPARTED. WHEN CLRED; THE CAPT BEGAN TO TAXI FORWARD AND I BEGAN MY BEFORE TKOF FLOW. AS OUR ACFT TAXIED ACROSS THE HOLD SHORT LINE; I LOOKED TO THE RIGHT TO CLR FINAL AND NOTICED THE ACR Y MD80 ON SHORT FINAL. YET AT FIRST; I COULD NOT TELL IF THE MD80 WAS ALIGNED WITH RWY 35C; THE NORMAL LNDG RWY; OR OUR RWY 35L. I TOLD THE CAPT TO STOP THE ACFT AND APPLIED THE BRAKES ON MY SIDE. I TOLD THE CAPT WHY I HAD STOPPED THE ACFT. HE LOOKED ON FINAL AND TURNED THE ACFT TO THE R AND TAXIED BACK ACROSS THE HOLD SHORT LINE. AT THIS TIME THE TWR CTLR ASKED ACR Y WHICH RWY HE WAS LNDG ON. THE PLT RESPONDED; 'WE WERE CLRED TO LAND ON 35L.' AT THAT TIME; THE CTLR TOLD THE MD80 TO GO AROUND AND CLRED US AGAIN INTO POSITION AND HOLD RWY 35L AND LATER CLRED FOR TKOF. HE NEVER SAID ANYTHING TO US ABOUT THE EVENT UNTIL WE ASKED HIM. HIS RESPONSE WAS 'AN ACFT WAS CLRED TO LAND AND ANOTHER IN POS AND HOLD.' I BELIEVE THAT WHEN THE MD80 CHECKED ON WITH TWR; HE ASKED TO LAND ON 35L INSTEAD OF 35C AND WITH THE DEP OF THE ACR Z; B717; THE CTLR FORGOT THE MD80 WAS LNDG ON 35L. WHEN WE NOTICED THE MD80 IT WAS APPROX 1/2 MI FINAL AND WITHIN 500 FT AGL.