A B727-200 CREW; BACK TAXIING ON RWY FOR TKOF AT TPA; DISCOVERED AN ACFT; APPARENTLY ON APCH TO THE RWY THEY WERE OCCUPYING.

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ASSIGNED RWY 18R FOR DEP FROM INTXN W-6. THE INTXN DEP WAS ASSIGNED DUE TO CONSTRUCTION. TXWY W IS CLOSED FROM A PORTION OF W6 TO THE APCH END OF RWY 18R. A REQUEST WAS MADE ON THE TWR FREQ FOR BACK TAXI ON RWY 36L FOR A FULL-LENGTH RWY 18R DEP. THE TWR CTLR ACKNOWLEDGED OUR REQUEST AND CLRED US TO BACK TAXI ON RWY 36L FULL-LENGTH DEP FOR RY 18R AND CLEARED US FOR TAKEOFF. PRIOR TO TURNING ONTO THE RWY FOR BACK TAXI; THE CREW CLRED THE RWYS L/R AND STATED SUCH PRIOR TO TAXI ONTO THE RWY. ACFT INBOARD LNDG LIGHTS; RWY TURNOFF LIGHTS AND STROBE LIGHTS WERE SELECTED ON PRIOR TO TURNING ONTO THE RWY. ON BACK TAXI 1/2 WAY BTWN TXWY W6 AND THE APCH END OF RWY 18R; THE CREW SPOTTED A DH8 MANEUVERING FROM WHAT LOOKED TO BE AN APCH FOR RWY 18L NOW MANEUVERING FOR AN APCH TO RWY 18R. WE WERE IN THE PROCESS OF ATTEMPTING TO CONTACT THE TWR IN REF TO THE DEVELOPING CONFLICT (FREQ WAS CONGESTED) WHEN THE DH8 CREW ASKED WHAT THE B727 WAS DOING ON RWY 18R. THE TWR IMMEDIATELY ISSUED MANEUVERING INSTRUCTIONS TO THE DH8 AND ADVISED US EXPEDITE AND CLR THE RWY AT THE END AND CANCELED OUR TKOF CLRNC. THE DH8 ELECTED TO MANEUVER BACK TO RWY 18L AND LAND. UPON CLRING THE RWY; I CONTACTED THE TWR CTLR AND RESTATED OUR CLRNC AS ISSUED AND INDICATED THAT WE HAD BEEN CLRED TO BACK TAXI; ETC. THE CTLR ACKNOWLEDGED OUR XMISSION IN AGREEMENT AND THEN ISSUED A NEW DEP CLRNC. THERE WAS A CHANGE IN THE INFLECTION OF THE CTLR'S VOICE AFTER THE RECOGNITION OF THE OPERATIONAL CONFLICT.